You can't test flow using a pressure gauge only, but I'm sure you know that.
Best way to get the oil hot is to bring along a jumper hose and loop it into two couplers of the same valve. Flow oil through this hose. Then deadhead another remote. This should force the pump to pump full flow and full pressure. This assumes the tractor is not MegaFlow. If it is MegaFlow, do this test using remotes # 1 and 2, or 3 and 4. Do this until the filter gets hot to the touch. That should be good enough.
I believe that standby pressure on those rigs was around 2750. Without a flowmeter though, you have no good way to test pump performance.
Not sure on the drain plugs. If it were me, I'd drain the oil and pull and check the suction screen.
It's been years since I've looked at codes on those things. I'm not sure whether overheat codes are permanently stored or if they're lost on a battery disconnect. Wouldn't hurt to have the dealer plug in and take a look if he has the tool. Absent the tool, a lot of info can be gleaned from the dash cluster, if you have a manual nearby to interpret the numbers.