Power Surge: Yeah I know. I guess I should add some countermeasure for that... maybe Hamper for the said turn.
Loop: Lasts until the creature dies, and it is pretty much of a threat with this. Still, PU costs 1 more Aether, but it lets you have twice the creature, not just twice it attacks!
Ingrain: Yup, +3/+3 per turn, but the Forest Spirit would have to grow once before. With the upgraded ones it'd be better. Or just for any creature to improve it a bit.
Endure: I guess it could be potentially bacon-saving when the enemy would be trying to kill one of your crucil creatures, but.
Hermit Spirit: I lol'd. 95% deaths in this game comes from enough damage in one turn or fast effects like Devour/Mutation. This one is also a real bacon-saver, as it gives a creature a second life.
Subversion: According to your logic, Sundial makes the creatures never attack, as it is a permanent and has Stasis. The effect occurs every turn, but the permanent lasts one turn. I can't make it a spell, as spells are... well, one-time effects, not something that works continiously.
Ice Absorb: See? Lots of strategy involved with this card. Would you rather make it wait for some time but get +6/+6 on it, or have it attacking now? Should I play this Iceshield?
Prune Eater: Nope, this is poisonous farting. Also, how would you be using this creature? Entropy + Lifesplash is used by Mutations so heavily they couldn't go together, and...
Mercury Blot: How would it be useful against Adrenaline? It's useful as it can't be Mutated or Stenched (see above). It can't be Adrenalined as well, for it is a Life spell, and that's certainly a worse thing. Still, a hard-to-get-rid-of creature and with Venom.
Wererat: It costs three turns to make it Lycanthrope. Can you afford it?
Shadow: Yes, it's a real cheap attacker, but brittle, and if it survives a turn, it's cheaper than an Elite Immortal and with same power. It depends entirely on a decision when to play it, with the preferable time being when the enemy has no creature control. A Precognition could tell you when it is.