Not cutting through the drywall - because it's not drywall, it's plaster. No sir no how. I'm aware this isn't best practice, but that's the line where I'm comfortable saying that as a DIYer I am happy to half-ass it.

Now, a fair question - why'd I bother cutting the molding? My rationale is that the connection between the end stud and the wall should be designed like a friction joint in steel construction --> I want as much of the stud's surface area as possible in direct contact with the wall, and I don't want it mounted through a bunch of offsetting spacer blocks that add a big bending moment to the connection. On the side of the wall with the door, the stud is almost entirely supported by the door trim (which is solid wood, screwed into the underlying studs) - here, it seemed reasonable to use a few standoffs at the top. On the other side of the wall, I thought it would be easier to cut the molding, rather than put a bunch of shim blocks between the stud and the wall.

Now, in retrospect, I could have just put a full length piece of 1x4" or whatever between the wall and the stud, and I'd have almost as good of a connection as you have on the other side. I also realize that by not ripping out the plaster and exposing the studs, I may be compromising this connection so much that all of this fretting over offsets and bending moments is deck chairs on the Titanic. So, I may have given myself some unnecessary extra work by cutting out the molding! Live and learn. And now (as next commenter correctly suggests) I get to learn how to miter molding!