Board Thread:Manga/@comment-32377911-20170628234245/@comment-1327106-20170629001014

Eren is the worst person from Reiner's perspective, because Eren is short-tempered and unlikely to be reasoned with. Someone else Reiner might have been able to take aside and talk to, but Eren is a ball of rage who hates Titans and also (more specifically) hates Reiner and Bertholdt for what they did to Wall Maria. Since Eren has the Coordinate, discussion is no longer an option and they have to take it by force.

I think at one point in the Uprising arc Hange calls Eren the personification of a rebellious teenager and says that the ones in power (Rod Reiss) won't let him keep his scream ability if it can be transferred. They'd want someone more controllable.

As for Marley attacking despite the unleash the Wall Titans warning, I assume they were either 1) willing to call the king's bluff or 2) desperate enough that they were willing to take the chance. Or both.