My only MM experience is with a M5 (and not much at that). What did you do about the "melted relief valve"? Sounds to me like the pump is deadheaded for some reason and the oil was going past the relief valve causing the heating problem. I would make sure all valves are in neutral. The M5 has very little hydraulic oil capacity and expect yours is the same, so it wouldn't take much to build heat fast. I don't think the M5 even had a relief valve, or least not an obvious one. I don't recall anything on the PTO operation.
If no one here can help, try on YesterdaysTractors.com.
Kim