Marking his edo before he sees kamui? Not happening. The kunai are marked, not prepped, he either has them on him or they're scattered around the battlefield within range, either way they only provide a small way to move around until they're destroyed or he just get's warped with the kunai in hand.
Danzo's seal was hidden until it activated, the marked kunai are connected to the users chakra so they must emit chakra. & FTG is a s/t ninjutsu.
The warp will be saved for last, if tobi's kunai are scattered around the field then obito needs to take them out. If he has them in hand then he needs to throw them or he simply keeps them. Either way obito has plenty of ways to destroy marked kunai so once they become useless then obito warps him. FTG being his escape route is his only way of effectively analyzing obito's kamui, without it he loses.