We finally managed to sneak the van back home to my parents' place, and it's not looking good. It's really starting to look like the engine computer's shit itself now. It has, in my mind, all the symptoms of a blocked catalytic converter (the diesel mechanic who undid the EGR delete insisted it didn't have a cat), except that if you unhook the battery for ~30 mins and reconnect it, the problem goes away.
It has a foul smelling, smoky exhaust, very little power with almost non-existent throttle response, and no turbo spool. Unplug the battery, and it runs great until it decides later on to act up again. Leaving it sit overnight with the battery connected doesn't solve the problem.
So we're basically done driving this Transit. I think it's time to start looking for our own vehicle and doing this thing properly.