I am not well versed on this unknown engine, but here is something to think about. If you look at the appropriate circuit scheme, there must be a part that controls your Maximum RPM. The Limp Mode is suppose to reduce RPM and Diesels do run at lower RPM than gas engines. Often an engine goes into LIMP when it senses overheating. On some engines, cylinders are deactivated and the dead cylinders are turned into cooling pumps.
Puzzling is the Glowplug warning. This can mean you have a dead short in some glowplugs. This easily can explain starting problems, but your engine will heat the glowplugs for probably 2 minutes or less and then they are off for a cold motor. Even restarting the engine when it is hot will not activate the glowplugs. So I can't see the glowplug problem causing a Limp Mode once you are underway.
It could be engineered to work this way though, and you need to refine the info about the Year, Model, and engine size.
I think I would try to change a temperature sensor, and pull the glowplugs out and test them.