Okay... it looks like it is time for some answers.

1. Why would Lily consider going on anything with Potter after learning that she has several potions and curses put on her, most likely by said man? 
She is still under the influence of the potions, so she still has "feelings" for James. While she is under the influence of potions and spells, they wouldn't create such strong emotions from nothing. They simply magnify the feelings and emotions already there.

2. Why would Severus support that?
Two reasons:  he knows the boys hate James Potter, so having him go along on the zoo trip serves as punishment for them. The other reason is that having Potter go in his place gives Severus time to brood a bit, and to brew the nullifier potion.

3. Shouldn't [Severus] be worried about Lily being coerced into even more traps by James? And made to drink even more manipulating potions?
Well, first of all, James doesn't know that he has been found out yet, so he isn't suspicious. IMHO, he wouldn't have any reason to think he needed to add more manipulative measures to what he has already done. However, even if he had slipped in more potions for Lily to consume, Severus is back 'home' brewing a Nullifier Potion, which would nullify any old or new potions in Lily's (or anyone who drinks it's) system. Problem solved.