It does not make any difference whether you use incremental or absolute form.
Fractional reserve applies to currently occupied LUN size, so reserved space will not increase initially after increasing LUN size (because no space is actually consumed). But of course if host later writes into this space, fractional reserve will increase.
Yes, I am sure. FR is by definition space reserved for overwrites, which means - amount of space currently occupied. If you have fully thick provisioned configuration, LUN reservation takes care of allocated size and fractional reserve takes care of currently occupied size.