Oh, I am having trouble with that too, but not just for the evolved form but the others as well ;-;

I am loving the questions since it reminds me of the things I need to think about, keep asking/suggesting things XD

Depends on the rules of the battle, but usually it is just one summon per battle. If it is a multi summon battle then yes, the next summon will come out if the first one is knocked out.