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1. Eventually no. You won't need to start over for new updates. The last few have changed some early mechanics and triggers, so if you reload a save it will break some interactions as they are initialised at the start of the game. 

2. The 9.0 update will yes. It will check each time fights happen to determine the tallest. 

3. Once the game is finished yes, I'll write up a guide for the endings and requirements for unlocking characters / special packs

4. In terms of raw power, Demons are probably the best. But be careful of your health since most Demon cards will use health as well as mana. Otherwise Fairy cards received a buff. 

5. Most endings now trigger at 8000 or more height now and Valeria's will trigger at 10000. 

6. For 9.0 you will need to start again I'm afraid as I changed A LOT. This should hopefully be the last time though. 

I dunno about the Patreon update, but with Succubus being locked, I'd look into Gladiators. There's some CHEESE you can do with the Forced Retreat card, which spawns two copies of Fleeing Gladiator in your deck. It barely does any damage, but its Withdraw effect causes you to recover one MP... and those effects go off not just when you return a card from your hand to the deck, but even just reshuffling the discard pile into the deck will trigger it. So you can quickly get to a point where you can just play nothing but Fleeing Gladiator on loop and go infinite. (It'll still take forever to actually kill the enemies, and the Gladiator card that says it buffs Gladiator's damage is bugged and doesn't actually work, but if a particular foe is really getting under your skin, this'll take them out.)

I also underrated Zombies for a long time, but there's some similar MP Cheese with the Zombie Corpse mechanics. Playing a Zombie Corpse adds some Human Corpses to the deck, and when those are sent to the discard pile, either via Mill or discarded from your hand, you recover one MP. The cards combo nicely with the Galactic set, which exiles cards from the discard pile to fuel effects. In particular, stuff that adds Galactic Spawn Eggs are a go-to; they lead you to summoning a Galactic Devourer who, when played, exiles your entire discard pile and then permanently gains that much damage. Spend the first few days sandbagging and stalling fights, to get that damage boost ridiculously high, then run a tight, low-card deck that focuses solely on getting a Galactic Devourer in hand ASAP.