It seems like firtPageCount and firstPageImages work the same. I tried both with the same results.
It worked just like you described, and I was able to make the first and successive pages display the correct number of images, no matter if the first or successive had greater numbers.
And as you described, it breaks the navigation. The navigation seems to account for one less additional page. For example, if you end up with enough images for the first page and a second and third page, the navigation will only take you to the second one. Even though the 3rd one exists. So the next page count seems to get screwed up by 1.
I'll see if I come up with a hack.