"How much do you feed your adults?"
Since you can´t overfeed an ig, you should offer as much as it will eat. If it eats ALL the food in the bowl, offer some more until it leaves some in the bowl.
"Do you feed them daily or every other day?"
Daily. Just like you would do with a baby ig.
"How active are the adults? Do they need daily exercise or prefer to bask most days?"
Adults are less active than babies and juveniles, and they like to bask most part of the day but they still need daily exercise.
"In summer, temps here are around 85-90 and humid, would an outdoor enclosure be okay so he/she can have some sunshine?"
Yes, it would be nice. But I´m sure night temps are lower. If it gets under 70º F you should bring it inside. Night temps should be 70/75º F. So if the temp gets below 70ºF at night, bring it inside.
"Winters are cold and dry,and it would be housed inside. Im just curious how would I go about keeping such a large enclosure humid enough for it? Would daily misting be enough?"
Daily misting is necessary but not enough. You can give it a bath (15/30 minutes) everyday or once a week with warm water. And also put a large water bowl in its cage. I put two, one for drinking and one for increase the humidity.
Hope it helps.
Later,
Dan