Yes to the first, a possible yes to the second.

In the database, go to the "Battle Event's" section of the Monster Groups tab and look at the Battle Event's box. Using the trigger "Switch [XXXX] ON" you can use the "Change Party; Add Member" event. Likewise, you can you the "Turns Elapsed" event to remove the summoned character. Or you could just remove that character if it's health reaches a certain point.

For the transform one, I'll have to do some research. Nothing's impossible, really. Just very difficult to do.

EDIT: Possible to "transform" characters, but not monsters.