I stuck with light armour through the entire game. The amour really doesn't make too much of a difference from what I've seen; on the harder difficulty settings you're pretty screwed if you get hit no matter what armour you're wearing lol.

It's definitively noticeable but not too bad. I'm using the Ursine (bear school) witcher set. But I do have quite a lot of points into my sign spells so the regen of my stamina somewhat balances.

I used light armor for about 80% of the game then switched to heavy armor. Now I'm using medium witcher armor and for the story I'm using heavy witcher armor. I prefer medium because it's a good balance but heavy is nice in addition to quen so I can tank hits on death march/ blood and bones.

It is true, bear school is in Skellige.

No the perk in the character menu. The one in the 4th skill tree.

I've been investing in magi and feel like my Mage but my gameplay has been pretty much swordplay except for using shields (shield is life) and mashing then dodgin and I occasionally get hit so that shield there but I play aggressively. Once I learned the armor thing now I'm spamming spells . I used to just apply shield and when overwhelmed just roll forever.