I can tell you that I wouldn't refund the money either. And I wouldn't pay you to have someone else go through it. I say that because most engine builders think their [censored] don't stink and their way is the only way. Doesn't make MM right or wrong. It's hard for me to believe that MM is as incompetent as is portrayed here. That would mean that the guys at MM can't even read a micrometer. I don't even think that for a nano second. You keep saying that the oil pressure was low. What was the oil pressure? What oil and grade did MM use on the dyno? Did you ask them what oil to run? What oil and filter did you use, as that has a huge effect on pressure.

As for the stuck valve, that can happen to anyone. I find it hard to believe that MM didn't size the guides. I would suspect a slow timing issue.

I couldn't care less what MM says the clearances are. You seem like a pretty sharp guy. Take it apart and measure them yourself. Then post what you have. Unless you have the correct dial bore gauge it will be hard to measure the ID of the guide but some shop near you could do that.

MM has already publically stated they would fix it for free. Print a copy and let them fix it. If they hold the engine for ransom you would have an easy court case.

EDIT: just re-read the OP and he is saying 15-20 psi oil pressure at 1200 RPM. To some, that is not low. For me, it's too low.But that still doesn't say if the oil was too thin or the oil filter was junk. Both of those will cause low oil pressure.