5100 grafts would be considered a very high number / large session. Your concerns are justified.
5100 grafts would be considered a very high number / large session. Your concerns are justified since the harvested hair will not grow back from the donor area.
Overharvesting of the donor area
Yes, from the pictures it does look like the donor area is overharvested. The single best solution is scalp micropigmentaiton (see below) which makes it look great
Safe Donor Area
I cannot clearly tell from your pictures the extent of the FUE extraction area but if it approximates the shaved area it is not appropriate. It is impossible to get 5,000 FUE grafts from the safe donor area. You can get 5,000 grafts but a great number of them may be programmed to be lost from male pattern balding.
Overharvested donor area?
You may very well be right: that's a lot of harvesting, and over harvesting seems to be more and more common. But I t's impossible (for me, anyway) to assess what your donor area looks like from the photos, since RealSelf does not provide a means for enlarging them. The only thing I can say is, time will tell. Also, I hope you don't go on to have any significant hair loss from your scalp, because there will be a limited number of donor follicles with which to replace it. Thanks for your question, and best of luck to you.
It is possible
to harvest that many grafts? they must have harvested/counted individual hairs. I guess it could be the # of grafts but that definitely sounds extreme. Why did you choose a place you could not do a follow up with?