The ability only activates if you deal that much damage (or more) with the card.
The opposite. Dealing that much damage (or less) gains the ability.
Impact 5: +1 Life per Life Left
Alright, so SoA, SoB, and DRs can all stop this gal from doubling Life.
Gentle 3: +1 Life per Opp Life Left
Please hit me with a DR.
I don't really like your +Life abilities Doombug, sorry. They're a bit OP and swingy, and coinflippy, for all it's worth. That FPC is 'just broken, again I'll use Kalindra to compare. SoA SoB and Dr all prevent, no, slightly deter her from doing what she does best, but against it all she is still unbelievably OP. Same with your hypothetical card, even against SoB or DR (let's say, Pussycats), he can still trigger the Impact with Fury, and 3 pillz now suddenly gives you +16 life gap on turn 1. Meh.
Nekro is okay. Considering he's in Nightmare, he won't automatically destroy any hand with an active Pussycats bonus.
I like Drig Volkrim, he serves the same role as Cutey in forcing pillz, but is even better because he doesn't need to pill himself, and the Vortex bonus increases the pillz gap even more. Great idea! As for Dhib, he's alright. I would use Naele over him though.
Is the max 10 referring to the max amount of life missing in order to proc the heal, or the max amount of life after healing? Seems alright, but no one uses a Roots 5* anyway