They were there last year while I was....so I understand the turning into a 13 year old girl thing.
I took my 3 month old baby, who fell asleep during the concert, so we left promptly after they finished their set for the first show of that night, so I can't answer the M&G part. They didn't announce anything, but I imagine you might be able to "bump into them" at some point. I expect its also possible that you could bump into them on a different day during your trip, because I imagine they use it as an opportunity to vacation at Disney with their families.
The show was good. They played some of their newer stuff along with the old favorites. MmmBop was particularly amusing, because their voices are much deeper now.