I would like to know the best way for me to get from Greeley, PA, to
New York City using only public transport for an overnight excursion,
preferably leaving after 7pm, and returning the next evening.

It looks like your best bet is the ShortLine Bus. It picks up at
Alice's Wonderland on Route 6 in Greeley and drops you off at the Port
Authority. The last bus picks up at 6:44 (a bit earlier than you had
hoped) and gets in at 9:21. You could return at 4:35 or 6:00 getting
you home at 7:19 or 8:54 respectively.


Here is a link to a PDF of the schedule.
http://www.shortlinebus.com/schedules/PA_Schedule.pdf

The Short Line bus homespage is simply www.shortline.com.

I'm not a researcher, just really bored at work. Hope this helps.