The rock would be a better footer than anything you would do... and it is likely deep enough to not matter on heaving.
I am not in favor of making extra work for myself. Sounds like you are!
Yep. If the frost isn't very deep there, and it sounds like it isn't, that big old rock will probably be the best footer you've got. Then again, it would sure suck to have to jack and shim 8 posts because the big old rock is so stable and steady that it doesn't sink like the rest.
