I'd never leave an oil filter on a motor for over 4,000 miles.

You never save money by scrimping on oil and filter changes.

As an amateur, I would. If I were a professional doing it for somebody else I would change it. Different standards when it is somebody else's vehicle.

Filters are changed to prevent buildup of debris and because the heat attacks the filter end pieces; both are a matter of miles rather than months. Passenger car filters are small; few hold 12 oz of oil, most considerably less. I usually get 4-8 oz out of my filters when I drain them before throwing them away. In your case, that oil is not very used anyway; the concern for the age is water/acid retention. Not enough of that in the filter to worry about.

I would not. The filter holds a quart of oil so if you do not at least remove it and dump it out you are just putting a quart of dirty oil back into the car. And since you have to remove it to dump it out you may as well replace it anyway. Oil and filters are cheap, new engines are not.

No. ALWAYS change the filter when changing oil. If you don't you will be leaving a liter of dirty oil in the engine to contaminate all the new oil you put back in.

Why change the oil?

At 2500 you could skip the filter change. Just do the oil way early by the way.