So I'm on the objective "Holding back the Horde", but all of the cavalry I'm supposed to destroy have taken refuge at the bridge to the East of Magetobria, which happens to be out of bounds, meaning none of my troops can even approach it. Is there a console command to turn off the barrier temporarily?

This has been a bit of an ongoing issue, they are working on it though, that much I do know.

Right now your best bet is to reload a save before that point and attempt to reach them before they retreat to friendly territory.

Is it possible to attack them with slingers from inside the boundary?