Good points overall. I would prefer to position and think of it this way for actually playing Ars Magica coming over: Expecting combat focus like in D&D+clones will likely result in disappointment. I would deemphasize the combat aspect altogether so they get a different game/flavor. They'll play MAGI, not fighters. I usually do that with the crossovers, so let them dig into what it is being a magi instead of traditional fight n' loot games.
That may include handwaving grogs, or not giving the grogs much opportunity to fight - choose your opponents.
Also, in the 1200s, I usually keep only grogs having maximum chainmail - no full-platers running around. Maybe the occasional knight (not even them have full plates at this time usually). So don't put forth combats with very high soak characters or opponents and you likely won't see much other than slight reduction on damage. Also remember magi never really have armor unless it's magical garments (limited to +3 soak or it's hard and uncomfy like armor), soak from Form bonus or active protective spell (which is usually not soak and mainly works against magic or as a specific ward).
You can have that be a later "challenge". Focus on having magic deal with things. The game is Ars Magica, not combat simulator.
Regardless, in the few hardcore grog combats we had in my last two campaigns (90ish sessions, 172 sessions and counting) - we're talking in the region of 5-6 fights where it came into play where magic wasn't the decider (but those were excellent). The combat was still faster than most D&D combats. Unlike D&D, damage scales with good attack roll, and it's 10% a 1 is rolled which quite often means good wounds or combat over regardless of soak. However, the soak tends to be what will let you survive, vs what will outright kill or maim you (kinda like in reality).
A hack you can do that we've used is making more sure this happens by changing that effect to instead of doubling on a 1, having a minimum result of 10 on the double (one method), or having each 1 equal to 10 and the roll and add (another method). A more linear increased result. Perhaps just for mundane combat, while keeping the crazy doubling 1's for magic.
We've had plenty of combats being quick decided by great roll. I.e. a "critical" more or less ends the fight. Or at least wounds opposition enough that it's quickly over a round or 2 after. Compared to endless rounds of decimating HP slowly in D&D - it's a breeze IMHO. Especially since already at light and medium wounds, the penalty stacks up fast (which is also why armor is GREAT to limit that). I would again recommend to play RAW first, or at least simualate a few grog fights (chainmail) and get a feel for it before doing surgery on the system.
I would also recommend running grog combat with the trained group (have a good turb captain with leadership and a strong vanguard!) vs trained or untrained group. Faster, more deadly, and let's you focus on the magi. We've had some fun fast slaughters with the elite well trained group vs rabble of bandits, dark faeries etc but also group trained soldiers. It's just one person rolling per side - i.e. much faster.