I'll be honest: even though I accept the ruling that TR locks on whatever BoM said first, it makes more sense to me that TR locks in on BoM and plays "whatever this card is" twice. The ONLY time this matters is on one-shot cards like Feast and Embargo, but when those are trashed they're no longer in play, so BoM is no longer locked in. However, BoM is in the trash now, so it can't lock onto anything because it's no longer "while in play" and therefore can't be anything.
And therefore?
A witch!...
I mean, therefore we let TR play that same action again, because it's better than letting BoM sit in the trash with no use at all.
(this means BoM as Fortress becomes a BoM again after being trashed and returned to your hand)
First of all, +1 for the witch.
I'm not sure, but we probably agree that with your ruling BoM would not lock-out of TR only because it gets trashed. It would still be locked as the card chosen for TR to double-play, as it's the card i chose from my hand.
It
would stop being the card you played it as, though, and would never again become another card (than BoM), as long as it is in the trash (and therefore not "in play"). But: You still
could play BoM a second time, choosing a card. And you
could play it as any cheaper card in the supply. You would not get any "while this in in play" benefits, as BoM doesn't
become the impersonated card, but you could
play it as it. This means any +2$, +2Cards, attacks, etc would still happen. That's not "nothing".