From the information LordHavoc has supplied, it seems as if single player will play out as an AI controlled version of this modification, but how exactly does Zymotic differ from such?

As the game is intended to be single player, are there any developments regarding complex entity support?

Lastly, how will multi-player fare in Zymotic? And again, are there any similarities to the previously mentioned modification?

I think only 1 map (the best of your 3) gets chosen and judged. I think thats how the proceedure goes... IMO you'd have more of a chance combining your efforts on 1 map rather then spreading yourself thin on 3. I am not sure whether is it specfic for maps, but keyboard keys are all functioning normally.