Hello, my name is XXXXX XXXXX I am here to assist you
Here is my concern: If you remove the cylinder heads to do valve guides and head gaskets, how do you not change the oil? I myself have never found a way to do that intrusive of a job and not change the engine oil.
Here is what can cause your high pressure situation:
Any form of debris that enters the oil pressure relief of the oil pump will cause it to stick and cause a bloated oil filter. Years back we had a machine shop in the area that did bead blasting on our cylinder heads. We went through several episodes of bloated oil filters and having to pull the oil pump apart and clean out the pressure relief valves before we finally determined that their bead blasting and perhaps their poor cleaning was the reason for our problem. I haven't seen that problem for many years.
In escense you already have plenty of evidence to go to small claims court with and I am quite certain that they will settle with you either before you get started or through insurance in court.
From your description, assuming you are accurate, there is no doubt that the work was not done correctly. The simple fact of not changing the oil is inexcusable.