15:45 is prime time for international arrivals at PHL. You may find the immigration line quite daunting. Since PHL is also a major US hub, you may also find a pile-up at the baggage re-check after customs, although on this I have no experience (since PHL is my home airport and I never check bags anyway).
I think you will squeak through, but be prepared for some anxiety, and be sure that you know what terminal you are going to so you can get through PHL with a minimum of searching.
I would not assume that AA/US will re-book you on the 18:00 DL flight if you miss the 17:15 US flight.