Suggestion: *Leave* it in Offset mode. The track sliders are then always providing a base level/offset and you never have to worry about which mode you're in.
If you have an envelope, then you have automation - the envelope mode is pretty much unnecessary*
Much simpler and I guarantee you'll be less confused in the long run.
* There's one case where you need to switch to envelope mode and back again. If you have an envelope and delete it, the "envelope mode" slider remains at the level of the envelope at the point in time you deleted it, so switch, double click on the slider to zero it, then back to offset mode again.