The 1500 scratches is very easy to answer, once they are buffed out they cannot reappear and it is always caused by the 1500 scratches getting filled with glazes from the compound and not buffed out, then it takes a couple of days or up to two weeks in the sun to burn off the glaze and the scratches will show.
When you are buffing metal temps of 100 degrees or higher, the compound is going to dry fast and this is very likely to happen.
Only thing I can think of is buff small areas at a time and on the initial cutting up the speed of buffer a little to burn out the glaze.
Maybe the panel should be wiped with a cold rag first? I don't know the answer if that would help or not.
To test to see if the glaze is gone is when you are done buffing, take wax and grease remover to the panel, if scratches show back up, then you know the glaze has filled the scratches instead of the compound getting rid of them.
As for the runs, I can't answer that one.