Combining Ascension expansions: tokens and boards
In my opinion:


I don't think there are any any interactions between sets to-date that would make this impermissible, but this might be inadvisable.  Consider that cards with set-specific interactions will become more ineffective as you combine sets with different mechanics (as the odds of useful interactions decreases, the value of those cards decrease).
Sure, but less tokens-per-player will presumably mean less cards and turns-per-player, so decks that excel in the short-term will win more often than decks that take a while to build out. I'm thinking "kill-the-monsters" type decks will be favored over "invest in the longer term"-type decks.
All of the Ascension games that I'm aware of can be played without boards altogether - they just suggest a physical layout for different types of cards.