Am I right that a lot (if not all) of the illusions (no matter what sense) that can be done with Imaginem, can also be done with Mentem by affecting the "common sense" (as per A&A p.31ff). Where Imaginem spells target "things" to create or change appearance, Mentem spells will target those that perceive the "things", but otherwise much of those Imaginem spells could be "transposed" to use Mentem instead, correct?
Mentem works well if you want a specific someone or specific group to observe some illusion, and, yes, you could make someone think they observe something. Imaginem works better if you want everyone to observe some illusion. So I wouldn't say that Mentem can do the same things, but rather something similar in many cases.
Let me try some examples, feedback is much appreciated:
change the orders a soldier/guard/... hears from his commander from unfavorable towards the caster to favorable would be MuMe base 2 or 3 (duration momentary if it's about a single command)
"Arrest him" would get "Protect him" or "escort him out safely" or something similar.
moving unnoticed by someone looking for the caster would be PeMe base 3 or 4
create an order as if it was given by a superior (i.e. sounding like the superior) would be CrMe base 3
I would say that qualifies as "Level 1: Make a minor change in a person’s memory of an event". You still remember your superior gave you an order and that the order was related to that person. If that is not a minor change, I don't know what could be. In this case, if the superior is still present, he could issue a new order anyway.
I think that Perdo guidelines are not designed to do actually that. You can remove the "major detail" of your character being there from target's memory (PeMe 4), but he is still seeing you at that moment.
Perhaps it can qualify as: Level 4 Diminish a single mental capability in a person. The ability to see you in this case (not the physical part, but the cognitive function used to tag you as a person and identify you). I don't know how much did medieval people knew about brain function. That is possible within the modern paradigm.
That is just words. If you what the compulsion part you need to use the Level 4 guideline: Put a thought or emotion into another’s mind, and if you want the memory of the actual superior doing the ordering is Level 5: Create a memory in another’s mind.
Penetration is an issue in that most imaginem spells deal with the manipulation of images that in turn give off non-magical species which are not resisted. But for the most part, yes, you can have a lot of the same outcomes with mentem as with imaginem.