Have you tried cranking the engine over with the fuel solenoid wire pulled and the oil filter removed??
Although my chinese 554 is obviously a different design, It failed to build oil pressure after mowing 10 + acres at a property about 10 miles away.
I had hauled it on my 5th wheel- and it was fine loading it up for the trip home, but when I started it up to drive it off the trailer -nothing- 0 PSI
after about 4 times starting it up and still no gauge reading I removed the oil filter and cranked it over.
In about 10 seconds or so the filter housing shot oil all over the trailer floor- But I was relieved...
If you haven't tried this it might be worth a try.
Hoping you don't have to split a fairly new tractor.
also i can't agree with the dealer on a stuck bypass- that would just bypass the filter and send un filtered oil at regular pressure into the oil galleries- a completely broken spring in the pressure relief(regulator) might cause zero pressure but have never heard of that happening on any engine.