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This is up to you. If you are going to re-install the folders and files will be re-installed anyway.
If you wish to perform a clean install, removing all legacy Cakewalk files and CbB, the old clean instructions still work 
https://legacy.cakewalk.com/Support/Knowledge-Base/2007013372/Clean-Install-All-Cakewalk-Software.
Be careful with these or any instructions regarding removing Cakewalk software. The instructions call for removing the default VST2 plug-in path "C:\Program Files\Cakewalk\Vstplugins" which may be a problem if the path is used for 3rd party plug-ins. Move/backup the 3rd party plug-ins before deleting the folder.
Wasn't the "good" version of Pantheon the one released before 8? IIRC, it was multichannel with some extra controls the stereo version bundled with 8 did not have. Regardless, both have been long eclipsed but if you want the plug-in, an advanced install should get it.  Don't remember if it was updated in 8.5.
As far as actually installing SONAR, I start with X2 to grab V-Vocal.

may want to review the SONAR Plug-in spreadsheet from this post
to see what else only came with X2.
Run the X2a update to get the last version of V-Vocal.
Then run a full install of Platinum.
Using the SONAR Spreadsheet again backfill with advanced installs of any other DAWs for plug-ins only. The old LP plug-ins were discontinued during Platinum. If you need those, the easiest way is an advanced install of X3- but, if necessary, I can look up which Platinum Engineering Suite installer has them. I know it has been posted before.
When done with legacy stuff...install CbB. Really, advanced installs of legacy stuff can happen anytime. CbB needs to be installed as the last DAW so that SONAR and CbB use the latest plug-in manager/scanner and shared plug-ins.

I guess the 64bit version of X2 did not get installed.
No big deal, unless V-Vocal is needed.
All the 64bit plug-ins including R-Mix may be installed using the advanced install option without installing the DAW core. Using advanced install for plug-ins only does not require re-installing CbB.
If V-Vocal is needed, a minimal installation along with whatever plug-ins would be required (video below). Doing this requires re-installing CbB to restore the shared components.

 
Running uninstall for 32bit SONAR does not affect 64bit installs but may affect your ability to open old projects.
Projects using 32bit plug-ins that have no 64bit version may load with the plug-ins missing.
If the 32bit VST plug-in is available, the DAW will load it in BitBridge. 32bit DX plug-ins cannot do this so go missing if there is no 64bit port.
 
The 32/64bit project port loop
Prep the project in a 32bit DAW with plug-ins that are known to have 64bit versions. Remove and replace all 32bit only DX plug-ins.  Load the project in the 64bit DAW and verify, Repeat  
 
Now if it's an R-Mix replacements you are looking for.... that is something else entirely.

As you figured out Aud.ini is does not affect the MIDI driver mode.
 
So how to figure out where this value is stored.
I could not remember but since Aud.ini is not a contender this usually points to the registry. 
There are two possibilities:
HKEY_CURRENT_USER\SOFTWARE\Cakewalk Music Software\Cakewalk HKEY_LOCAL_MACHINE\SOFTWARE\Cakewalk Music Software\Cakewalk The CbB specific values are stored in Core under these paths.
To test if this value is stored in the registry, I
Exported both registry entries using regedit Changed the MIDI driver in preferences Exported the registry entries again Used diff forcing a text comparison of the registry exports and  Determined the MIDI driver value is stored in CoreMidi in HKEY_CURRENT_USER\SOFTWARE\Cakewalk Music Software\Cakewalk\Core The values for CoreMidi are
0 for MME 1 for UWP  
 
If I had no way to figure this out, a clean install of the DAW includes removal of the registry entries. This would solve the problem too. Be aware that step nine of the clean install process may result in the loss of 3rd party plug-ins. Consider moving the VSTplugins folders out from under these paths to a safe location before performing this step
 

Step recording  may help

Cakewalk support e-mail is support@cakewalk.com - use this for Cakewalk by BandLab (there is also a link in the Cakewalk by Bandlab tab as the top of this forum to the support page too).
BandLab support e-mail is support@bandlab.com use this for the web-based DAW called BandLab
The DAWS are made and supportbed by two different groups working for the same company.

Access to video start time requires the old DirectShow video engine. 
Here is how to set the default video engine.
If the video format is not supported by DirectShow, the DAW falls back to the Media Foundation engine.
 

While not the same hardware, it may need the same CbB config tweak as other Avid interfaces.
See 
 

Try
Move Now to the expected end of the project  Select ALL Select From=Now Enable Ripple Edit All Press the DELETE key Turn Ripple Edid off

If the program hangs on the splash screen 

it may trying to discover the audio drivers available to the DAW
If you are using ASIO driver mode, you may want to verify there is at most one manufacturer supplied driver for each audio interface and no generic drivers are referenced in HKEY_LOCAL_MACHINE\SOFTWARE\ASIO
If you are using a Realtek audio chip do NOT use ASIO driver mode, the driver is not suitable for DAW use.
 
It could be a problem with the MS redistributable libraries, here is an installer to fix this
 
Finally, if the problem is not ASIO driver or MS lib related, you will likely need to forward a dump to support for diagnosis. Here is information about how to proceed
 

The message indicates you are starting CbB with the SHIFT key held down invoking partial personalization.
For safe-mode hold down the SHIFT key while OPENing a project.
IOW...
SHIFT while starting CbB = partial personalization
SHIFT while opening a project = safe-mode.
 
Alternately, physically moving the offending VST3 out of the scan path should allow the projects to open with a missing plug-in message.
 

Yes, PTN files and the ARP files created from them were made using Project 5.
PTN and ARP playback was added to SONAR but it never got the editor.
I have no idea if the original file is still available on the internet or not.
I do have the ARPs installed but it came from a large collection of customizations that get copied from machine to machine. 
Found a copy of "Alesion ARP Presets.zip" in a snapshot of one of my old PCs incoming files folders made in Aug 2007.
The file is now on my Google page under presets.
 

This happens when re-installing without cleaning up the registry.
Here are the clean install instructions https://help.cakewalk.com/hc/en-us/articles/360034066393-Clean-Install-Cakewalk-by-BandLab

The piano roll works at the track level.
To see one lane at a time, mute the clips in all the other lanes and enable "Hide Muted Clips" in the PRV View menu.
To see one clip at a time, mute all the other clips in the track and enable "Hide Muted Clips" in the PRV View menu.

By default, ALT+F4 has no global binding in CbB.
If you have unsaved changes before export, consider saving before starting an export.
To interrupt an export, click the x next to the progress bar in the transport

It may not respond immediately but ultimately should show the dialog in the image above just before stopping the export.

In the project that does not expand selected track take lanes, is auto track zoom enabled? If so, disable it.

I don't believe so.
Not all functions are exposed for (re)mapping.
That said, AutoHotkey or something similar is a skill worth picking up.
For example, here is the AutoHotkey script that maps the (currently unbound) F2 function key to CTRL+i in CbB (and only CbB)

When CbB has focus, press the F2 key and CbB receives CTRL+i.
 
 

The short answer is No.
There is no way to alter the default track to either preload PC settings or swap it for a track template in CbB.
I created a shortcut using AutoHotkey to load a specific track template, in this example, a template for guitar. The script, bound Windows+Space, opens the template import dialog then navigates to my custom template folder then loads the guitar track template.
 

To import multiple clips, one after the other, into a track starting at 0
Select clips (in the gif below, I used a selection from the media browser) Click-drag the clips to the area between the track header and the clips area (AKA track scale)  Hold down CTRL+SHIFT Release the mouse button to drop the clips in the track.
 

Depends on which preset manager is used to save the preset.
 
The Cakewalk preset manager stores its data in the registry under
HKEY_CURRENT_USER\SOFTWARE\Cakewalk Music Software\ActiveMovie\Presets\
using the plug-in CLSID as the key.
 
As noted above preset managers integrated into plug-ins most often store presets somewhere on disk.

To send MIDI data to an audio FX plug-in:
Open the plug-in UI and "Enable MIDI Input" in the plug-in standard header VST2/3 drop down

then add a MIDI track with the input set to your controller, output set to the plug-in and enable input echo (until the data is recorded).

If the image shows the "Sort by Category" layout there is definitely a problem with SI-Drum Kit.
If SI-Drum Kit were available, the layout would show a folder for Drums.
The section below Default (TTS-1) and above the Bass folder holds a list of up to 5 of the most recently used plug-ins.
The list indicates SI-Drum Kit was available at some time on the PC but the folders below the list indicate the plug-in is no longer available.
It is impossible to say with the information available what happened. 
If the machine has not been restarted in some time, this may be a good time to do it.
Running a VST reset from preferences may pick up the plug-in again. The Reset button is in the scan option section.
If not, then re-installing the Studio Instrument suite may restore the plug-in.
If the Studio Instrument Suite was installed using the Cakewalk Installer, then the Studio Instrument Suite installer should be in a Cakewalk folder in your Download directory.
To get a copy of the Studio Instrument Suite installer run the Cakewalk Installer available at the bottom of this page.

Right-click between the clone track button and the track control drop down to add it back

The Sonitus preset manager uses ini files to store presets.
By default, the ini files are located in "C:\Program Files\Cakewalk\Shared Plugins" and user have read-only access to them.
In order to save presets
right-click on the ini file(s) open properties from the file context menu edit permissions allow Write permission for users  

Is "Show Note Durations in Drum Grid" enabled in the PRV notes menu?
The name "drum maps" is a little misleading as the feature is a general-purpose tool for note labeling, translation and signal routing.

Use Windows explorer to create the folder(s), then drag the clips into folder(s) in Windows explorer or the folder(s) in the media browser.