Absolutely. The trick here is to play the long game. Corrupted Marth will eventually lose all uses of his Rapier, in which he'll swap to the ineffecitve Iron Sword. There's a reason why Nyna is there after all: To keep Hardin and Camus alive. If Hardin dies, you automatically get the canonical ending.
However, I think that the third ending behind beating Marth is endless void, where you're forced to skip turn after turn, with Hardin all alone in it. If the developer of this game can confirm that this softlock map is already the third ending they were talking about, that would be nice.