There's a big difference in the wording between Bella's ability and Demetryo's, though.
Belladonna: "When Belladonna is summoned, draw a card."
It's pretty clear her affect only triggers when she is summoned, which only happens once.
Demetreyo: "Demetreyo is killed at the end of the turn he was summoned."
Depending on how you interpret that, it's not that same as "When Demetreyo is summoned, he is killed at the end of the turn." If it was worded this way, there would be no question that he would stay alive after being brought into play by EP.
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