Yeah, that's a hassle.
"..flips your page view at the beginning of the "Next 100"
rather than on the Keep'ed post"
Here's a work-around for it:
When you're in the Next 100 mode and you see a post you want to keep, instead of clicking "keep", first put your cursor in the "Post #" box and hit return. That takes you to the post itself. THEN, hit "keep".
After that, just hit the backspace button, and it takes you right back to your nice warm spot in the middle of the the Next 100 page.