I don't know what you mean by "merge", but you should place this after Soundtrack Manager in your plugin list if you use it.
Viewing post in Soundtrack Layering for RPG Maker MV and MZ comments
I guess you could do that. There may be a function or two that exists in both plugins. It's not really a scenario I support.
And yeah, there's no way to adjust the volume of the layers without my plugin, because base RPG Maker wasn't designed around having layers. (Except the Fade Out command, which I did overwrite to fade out all layers at once.) And... I actually realize there's a scenario involving adjusting layer volume I never tested. I'll... just check that real quick.
EDIT: Tested it. Found a small issue. Updated Soundtrack Manager to fix it. Thanks for somewhat indirectly making me realize it might be an issue. *ahem*
If you try to play the current base track at a different volume, either through Soundtrack Manager or the built-in PlayBGM command, it will change the volume only of the base track. If you use PlayBGMLayer with a layer that's already playing at a different volume, then it will adjust the volume only of that one layer.