As long as your United flight isn't terribly late, you should be fine. I consider 90 minutes a reasonable connection time (as in you won't have to run with your luggage) for that airport. It's all one building and the distances aren't too great for the time at your disposal.

I think you should be fine.

1) Yes, it's plenty of time (barring any extensive delays from Philly, of course)

2) Not at all difficult...Athens airport is very easy to navigate and all of the check-in desks are in the same area...after that, just follow the signs to your gate (generally domestic flights leave from the "A" gates, if I remember correctly, but it's all in the same building and equally easy to reach).

Thanks everyone.