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.