I guess my question for the experts here is: what information should OP have tried to gain and document to either:

1 - convince himself it is weather?
2 - prove to UA it should not be classified weather?

I think this is not an unusual situation. If the GA had been told that it was weather at XYZ affecting the crew, then one can look up the incoming flight, etc. and make a good decision before waiting for 2 hours...

If a crew or weather problem affected another flight and UA decides to pull the crew or aircraft from the flight OP is on, would that be classified as weather?

The lack of transparency and communication in this case makes people wonder and it is understandable. Sometimes the GAs are very open and share all the information.