Here to Help

Dragon Roost is at play. I control the other battlefield with a hidden Here to Help. If I play Here to Help and I pay the 2P additional cost for Dragon Roost, am I allowed to play a Dragon there? If yes, why? If not, is the 2P wasted?

No. The 2P would be refunded.

Dragon Roost is NOT a replacement effect. It only adds to the valid locations that the unit can play to (355.2.b.). However, because Here to Help played from hidden, the chosen dragon MUST be played to that battlefield (811.1.d.3.).

By the time we get to step 5), we see this is a play that would be illegal (358.3.), This should unwind the process of play for the Here to Help dragon unit and refund the 2P.

Arguably because the opponent has not revealed any new information, a rewind could go all the way back to when Here to Help is resolving where the player could play another unit. But that’s a can of worms for another day.