Okay, so I like everyone else’s comments, but they feel complicated with what I came up with:
Harry convinces himself of #2 enough to say it in parselmouth.
Harry says “I think I understand the prophecy you’re trying to avoid, and I believe killing me makes it happen. I would say more, but you’d probably use it to kill me” in parselmouth.
Okay, so I like everyone else’s comments, but they feel complicated with what I came up with:
Harry convinces himself of #2 enough to say it in parselmouth.
Harry says “I think I understand the prophecy you’re trying to avoid, and I believe killing me makes it happen. I would say more, but you’d probably use it to kill me” in parselmouth.
Harry stays silent.
You should be more explicit in Step 1.