My name is Scott, I am an experienced small animal veterinary surgeon and I will be very pleased to work with you today and will try my best to answer your question to your satisfaction.
I have spent some time reading your question through thoroughly and I have a couple of points to make, the first one is that it can be quite inaccurate to determine how many puppies are present just by feeling her abdomen, many vets have been caught out doing this. X-ray and ultrasound would be better of course but you cannot normally keep on using them.
The second point is that I doubt the Pancur or change in diet was related to events, Panacur has a very high margin of safety and I personally would have no hesitation using it in pregnant dogs.
I think it probable that she has indeed had an infection of some kind but is now thankfully recovering and on the mend. At day 50 at the moment as long as she seems OK then the best plan would be just to wait until Day 63 and see what happens. Yes she could have a dead puppy and yes she could abort it before term but as she is improving you have nothing to gain by not holding on that bit longer.
Of course if things change and she starts to show any symptoms or you become further concerned then intervention may be necessary but quite frankly it would be best if you do not have to go down this road/
I am standing by for your comments ...