I haven't been able to purchase my park tickets yet but my FP date has passed, will it be impossible to get good fastpasses once I have purchased the tickets?
Most likely you will not be able to get Frozen Ever After or 7DMT but everything else should be available.
Depends on how many days you are away from your trip. You will also most likely have to stick with FPs for afternoon/evening - morning passes go first (because people want to try to get a 4th day-of FP in the park) .
Once you purchase your tickets, and even after you make your initial FPP selections, keep checking MDE for availability of the FPP that you were unable to originally obtain. I just recently helped my brother and his GF schedule their FPP for a spontaneous trip to The World. With 3 days notice, and some persistence, they were able to get 2 FPP for 7DMT (read: I was able to score them FPP for 7DMT )!