I wanted to try making a deck around these three cards since it seems like they would interact really well with each other but I wanted to make sure of a few things:
If I attack with Dreadhorde Arcanist and exile a spell with Krark, the Thumbless on the battlefield, would the spell return to my hand if I lost the flip instead of exile? That seems really good if true.
Next, if I have Harmonic Prodigy and I cast a spell with Krark, the Thumbless on the battlefield, would I flip two coins? I think the only way it could fizzle is if I got two tails in a row. A heads and a tails would copy the spell and return it to my hand and two heads would copy it twice.
If you cast a spell with Dreadhorde Arcanist and the spell would move from the stack to your hand (or to a zone other than your graveyard), Dreadhorde Arcanist won't interfere with that movement. Compare with flashback and unearth (C.R. 702.34a, 702.84a), and see also this thread.
If Krark's first ability somehow triggers more than once for the same spell, you flip a separate coin when each such ability resolves. When Krark's first ability resolves, if you flip a coin and win, but the spell it refers to has left the stack in the meantime (whether because it moved to hand or was countered or for any other reason), you still copy the spell using the spell's last known information (C.R. 608.2h). (Note that the ability has no targets [C.R. 115.1d].)
If I attack with Dreadhorde Arcanist and exile a spell with Krark, the Thumbless on the battlefield, would the spell return to my hand if I lost the flip instead of exile? That seems really good if true.
Next, if I have Harmonic Prodigy and I cast a spell with Krark, the Thumbless on the battlefield, would I flip two coins? I think the only way it could fizzle is if I got two tails in a row. A heads and a tails would copy the spell and return it to my hand and two heads would copy it twice.
If Krark's first ability somehow triggers more than once for the same spell, you flip a separate coin when each such ability resolves. When Krark's first ability resolves, if you flip a coin and win, but the spell it refers to has left the stack in the meantime (whether because it moved to hand or was countered or for any other reason), you still copy the spell using the spell's last known information (C.R. 608.2h). (Note that the ability has no targets [C.R. 115.1d].)
EDIT (Mar. 5): Edited.