Summons during Party Attack (while blocking)

Yes, they can.

Yes, they will.

As many as you want. Or, more accurately:
You can play one Summon or ability every time you gain priority.
Every time a Summon or ability is added to the stack, the turn player gains priority.
When the turn player passes priority, the non-turn player gains priority.
When the non-turn player passes priority, the most recent item in the stack resolves and the turn player gains priority.
If non-turn player passes priority there are no items left in the stack, the game proceeds instead.