Do characters in WAIT still gain XP?
After over 30 hours of gameplay, I can confirm that all characters currently in your party receive the same amount of experience, even when in the back or not active.
I have been playing exclusively with Neptune, Plutia, and Peash in the front, and Compa, IF, and Vert in the back. Noire, Blanc, and all other characters still levelled up at the same rate.
All characters are now over level 56, except for a few characters who left the party in between and are now level 48, and another few characters who just joined.