Is diagnosing these motors guesswork?
No, not when you choose the right mechanic.
Mechanics that think
they know this engine are very different from those that actually do know
The guy that thinks
he knows it will end up charging you for his education. The "$3,000 for an o-ring problem" is the perfect example of this.
It really sucks that you are going through this, but to stop your wallet from bleeding all your cash:
A: Call your current mechanic right now and tell him to cease any more work on your truck.
B: Find a mechanic that already knows the 6.0
C: Call a towing company to pick your truck up from your current mechanic and deliver it to a mechanic that actually knows the engine. Try to get your truck back from them as cheaply as you can. You have already over-paid for his limited 6.0 education.
D: Have new mechanic verify that lifter(s) failed...if not..find out what the real problem is and ask how much it will be to fix it, up front.