I tried pretty much everything and now I'm fond of my adrenaline and decoctions junkie build
Abilities I use:
Strong Attack crit chance and damage boosted with Viper shool swords with their 75% buff to crit damage
Rend - the best skill in the game imo. Can't imagine myself without it anymore
One that adds 2% insta kill chance for each adrenaline points you have.
One that grants you one free adrenaline point whenever a fight starts. It also significantly boosts the rate you get your adrenaline points at.
"Flood of Anger" - uses all 3 adrenaline points if available when casting a sign but in turn significantly increase that sign damage and unlocks all upgrades to it at max capacity. Mostly used as a fun factor
But I'm kind of fond of it. Adds more versatility to my character and makes you not spam signs but use them when the situtaion is most fitting.
NEVER use quen
One that gives you extra toxicity point for recipes learned. - Stuff up on delicious decoctions!
One that makes you immune to overdose on potions. - Don't die of them.
One that eliminates toxicity gained by consuming potions in mere seconds. - Still have to use all the other potions at will.
One that gives you 1000 HP for each decoction consumed - It's +3000 HP in my case (9560 with all buffs)Tank it up, baby.
One that gives you additional 25% protection if you have a right oil on your sword - More tanking!
One that boosts the bonus you get from mutagens.
Axii - Dialouges and stunning humans stupid enough to go against me.
45 + 45 + 30 - Red mutagens
+900 HP from a green mutagen
Thuderbold, Tawny Oil, Blizzard, Maribot Forest - Potions I use most.
Northern Wind and Devil's puffball + Golden Oreol - Bombs I use most.
Mastercrafted Ursine Gear imported to NG+ (Not going to change it). Vitality runes to regenerate lost helth in fight and after. DM here, sick and tired of opening this clunky inventory just to eat another dumpling. Deflection enchantment - Damn archers!
Currently Viper Swords + the enchantment that buffs rend's range. Slice them up from safety.
Pretty much feels invincible. Even if going against enemies that are of a significatly higher level, provided I drink the right decoctions. My strong attack doesn't deal 5K crit damage, but it's at 4.2K. I think it's adequate.
To answer your questions:
Changing the armor type with enchantments doesn't nerf the armor stats. No worries there.
Well, devils's puffball + superior golden oreol - This heals you and damages your enemies, provided you stay in the poisonous cloud. Other bombs are mostly situational, like moon dust to make those damn wraiths flesh and blood for easy slicing.
As for the sign build, in my opinion - It's not worth it. The worst playstyle I tried so far. Low damage, mosly useless abilities, ugly armor, boring Igni that makes you resort to frying enemies that can be fried, a lot of them can't.