How does Unbound Flourishing work with Ice Cauldron's first ability? It looks like the copying & the exiling are mandatory, assuming you have two nonland cards in hand. You can cast both of the cards until you activate the first ability again, & use mana from the second ability to cast either or both. Is that correct?
How does exiling a nonland card from your hand work, anyway, given the hand is typically a hidden zone? Who verifies that you don't have a nonland card in hand? This dynamic seems a bit odd.
You have pointed out an apparent oversight with Ice Cauldron's first ability: the intent is arguably "...you may exile a nonland card from your hand..." (compare Flash as printed in Sixth Edition with its Oracle text). I have notified the rules manager.
Ice Cauldron's last ability allows the mana added due to that ability to be spent only "to cast the last card exiled with Ice Cauldron" (C.R. 108.1).
EDIT (Jul. 9): Struck out first paragraph since Ice Cauldron's Oracle text was updated with Dominaria.
Thanks! I guess it's really fine either way, as you can still cast the first card you exile even if you can't spend the mana from Ice Cauldron on it, but is odd to have to exile a card & how that works with hand as a hidden zone. I hope they do change it to a may.