My view would be that if you can still see oil on the dipstick, and only 15 seconds of engine light, then there's no reason to think it'd have run a bearing. It may have, but it's not a given.
If it cranks then that's another reason to think it's not locked up, although no guarantee. Also if you've had dealers look at it I'd presume they can tell just by listening to it if a bearing's gone.
Diesels are conceptually very simple engines. No spark plugs and therefore no spark needed. If they crank and they have fuel they should at least fire. You say it's cranking. That really leaves fuel delivery as the problem. As you say, there may be a fuel cutout, there may be other things going on.
Someone competent to do so could crack then injectors and add a gauge and/or other diagnostic technique to see if it's delivering fuel into the cylinders. If it's not, then you have a clear path of enquiry - it's not delivering fuel, there's a limited number of things that can cause that.
If it is delivering fuel (and if it's not firing, you should also be able to tell that from a strong diesel smell in the exhaust I'd assume - you might have to remove the DPF to smell that), then you're down to:
- no air. Weird, but I guess something could be wrong with the air cleaner. Someone put it back together backwards or something
- no compression. Again, something big has to go wrong to have no compression, and weird to have no compression on all the cylinders at the same time - it should still fire on three cylinders if one is busted
- it's not really cranking, or not cranking fast enough, and you're not experienced enough to notice. Most people would notice straight away if it's cranking but doesn't sound right - but if you're not really a tractor guy maybe it's cranking slowly / partially seized, and you're not recognising it
If I lived in the USA I'd give you $10K for it and take it off your hands. I find it very hard to believe it cannot be fixed.