A delf would never consider suicide, unless he's sacrificing himself for the sake of the tribe.
A delven ability to run is part of their identity. For them to put that ability under question is to betray themselves. When the choice between betraying themselves, or ensuring personal death comes to them, the thing that breaks that tie are tribal needs. If the tribe will suffer because they did not ride the wagon, then they will ride the wagon. But once it's done and the need for the tribe is gone, please understand, that they have lost themselves. They're no longer elves. They've died, for they lost what makes them elves. They still cant suicide, because it is absolute anathema for them, but they'd seek out every possible way to sacrifice themselves for the tribe, because … they're already dead. And it's not a matter of them being shunned, or exiled, or accused. There will be no need. They will be able to do this to themselves all on their own.
So if there is a mekillot on the ridge and you have 1% chance to survive running past it, or 100% to survive by riding a wagon. They will pick 1% chance, because riding a wagon means "certain" death. Unless … the selfishness of choosing not to die, will make the tribe suffer. In which case delf will do everything he can to assure tribe's well being, and only then worry about his eventual invalidation as a being.
At least this is how I understand it.
Overall. There is a myriad of ways to practice mental acrobatics to talk your delven self into being able to ride wagons. The fact is that it's against theme. It doesn't have to make sense, it's just part of the racial quirk and a player needs to play it out, instead of creating an 'exception' every time. It's not difficult to give a good logical reasoning why delf should be able to do it. It is simply requested by the theme of the game that a player does not do that, because that is the theme of the race.
