StudioNSFW

It really depends on who the songwriter is.  if it is original and destined for ANY  distribution, you really need to establish at least one publishing entity to capture publishing royalties (thanks to the extremely antiquated licensing model still embraced by the powers that be in the recording industry). Publishing royalties are typically almost the same amount as songwriter royalties.  Just about anyone who does publishing will have two or three entities - one for ASCAP, one for BMI, and one for SESAC - because you cannot have the same entity name with more than one clearing house. Many (all serious, professional, I-do-this-for-a-living) songwriters will have a publishing entity, or two, or three...or they work for a "house" that has that all set up already.
If someone else wrote it and/or published it, you can still use it for commercial use, you just need to make sure royalties are paid. There are statutory rates available from any one of the clearing houses listed above but you can almost always negotiate a lower rate (based on usage/distribution). 
You cannot copyright a previously published song or ANY performance, and copyright is completely independent of what production method was used to produce THAT version.  That is where ISRC codes come into play.  Don't worry about ISRC unless you are creating a commercial CD intended for airplay.  ISRC is a numeric string embedded in a .cda file (CD version of a .wav) that identifies the exact performance or iteration of a given song, and ties it back to the label.  It is used by the players in all radio stations to report "Spins" and that information gets sent to a company named Soundscan, who compiles the information and makes sure royalties are paid by the radio station to the song writers and publishers.  If you are not the songwriter or the publisher, you can still get mechanical royalties as a performer, which are almost nothing. I have gotten quite a few checks that were for less money than the stamp on the envelope needed to mail the check. 
 If you get with a major label, one of the contract stipulations will be that the label will get the publishing rights for all songs created during the term of the contract - this can be negotiated down to "for xx years" after which the publishing can revert.   
And, to answer the original question - you ABSOLUTELY  can use Cakewalk to produce commercial music, and they don't get a split on the royalties.

That's called punching. Yes, it's supported and smooth like buttah
MAke sure your recording preferences are set to overwrite
Select and set your in and out points
click the handy "Set Punch  points to selection" in the control bar widget. THis will enable the punch as well.
Go to town and fix whatever.  The offending section will be removed when you hit record, but record enable will toggle on when you hit the IN patch point. I'll usually "Roll in" a few measures before the punch point. 
The most important thing is to turn it back off when you are done, lest you recreate  my favorite "WTF??!!" moment...