You're thinking too much.
Try this track. Recorded live. MS on Guitar, Mono on Vox. This was an acoustic show so the mics were pulled back a bit (20"-24" maybe). I think you can use it to put your theory to the test.
What you say may be true, but I suspect that in a mono mix you WANT the vocal to sit up a little higher since you have no stereo information to create space for the vox above the instruments. The phenomena you're trying to avoid might actually be working in your favor.
(The track is from One Nightstand, Indianapolis, by Trina Hamlin)