By the RAW, does a supernatural ability, said entrancement, give a magical resistance in Mentem?
To the best of my knowledge, no.
Same thing here
No, but the reverse is true - Mentem would give a Form resistance against Entrancement.
No; Hermetic Magic/Parma Magica is the only non-Divine, non-Infernal source of MR.
A Hedge Wizard tradition with access to the Entrancement ability should probably receive the Immovable Object magical defense though...
I vaguely recall reading a section of an Ars Magica book suggesting that supernatural abilities might allow characters to resist magical effects, but I just can't seem to find it...
...other than a Might Score.
It was my thought. My SG houserulled the opposite, but i find it a little aside the MR process, so i will talk about it with him.
Parma, True faith, angelic/demonic powers, magical might, certain magic items/amulets of some hedge traditions, some magic items made by hermetic mages and more can give magic protection.
To say that only Parma outside might and divine/infernal intervention really really limited magic resistance in the world.
This said, most supernatural abilities are just abilities that give you power to do something. Entrancement is like a rego/muto/Creo mentem effect. This is not magic resistance in any form. The only ABILITY that gives magic resistance is Parma.
I thing that "a defense" refering to Magical Abilities mustn't be readed like Magic ressitance, it should read like a magical counter effect, like a Perdo Corpus effect owned with Shapechanger, or a effect of Auram versus the Whistle of the wind.
I think that this is a good clue.
Hedge Magic includes rules for Magical Defences, where Supernatural Abilities (and Hedge Arts) provide some sort of protection against magical effects. This is not Magic Resistance even in a limited sense; the incoming spell always has some effect, but it might be reduced in duration or damage, or require some sort of opposed roll to work as intended. Your SG might be using something along these lines.
There is an implication that Magical Defences are possessed by hedge traditions rather than individual practitioners; so if you just have the Shapeshifter Ability you don't automatically have a Magical Defence involving this Ability unless you've been formally trained in a tradition.