The Seventy Five Foot Tall Rock Giant

Indeed. Very few companions can withstand either a Pilum of Fire or Crystal Dart without taking a lot of damage. Being vulnerable to a different, low-level spell seems perfectly reasonable. After all, a magus' familiar will normally be fairly low Might so ...

Might 20 is still hardly invulnerable, but like I said, it's a judgement call for your saga.

I think that 5-10 is too low for a companion character, for a saga that runs for a long-time post-gauntlet, and if there is no mechanism for his Might to improve during play. So, I'd make it 20. You wouldn't, that's fair enough.

Although a familiar inherits his Master's Form resistance (if it's better than his Might) and would almost certainly be "protected" from being nuked out of hand by other Hermetics via the Code clauses prohibiting "deprivation of magic". Neither applies to a companion character.

Why is a magical creature dead once deprived of all its might points? Are there any RAW. I do understand that ghosts and demons vanish as they do not posses a real physical body... However, IIRC a Verditius Inner Mystery lets you strip an animal of all its might to use all the Might points similar to Vis. But I do not recall that these creatures are dead aferwards.
So I would use a house rule stating that once derived of their might, magical creatures loose all their powers (even those requireing no activation cost) and might even feel dizzy or loose their reasoning but they would stay alive.