do party members gain LP/level up when they're inactive?
With only 3 party members at a time seems like a pain to have to constantly be shuffling them.
LP yes. EXP no.
Equip golden amulets on all the characters sitting out since it doubles their LP. You can get them really early in the Mosphoran Highwastes from the shop seller there. I'm assuming that hasn't changed in the Zodiac version anyway.