1) You can spawn 16bit monsters on your spawn action - but after the counter hits "16bit" mark
2) They are lost. Potions or hearts are never "on the field"
3) During one whole turn (1st hero - 4 skulls of monsters - 2nd hero - 4 skulls of monsters - 3rd hero...until all monsters have been activated once) any monster can be activated just once. On the next turn, you can activate monster again.
I'm talking about the 2.0 rules.
1. There is no tracking board.
3. In 1.0 it works this way, but in 2.0 it sounds like they changed it.