There really is no room in between the tvps for the reverse servo. Maybe you could get a micro servo in there? Or if that would even work?
I never ran my Savage with reverse, so I am not used to having it.
I am sure you could put the servo on the outside of the tvps. I think the best thing would be to lock in the tranny if you really wanted reverse.

As for the a123 cells. I have stick packs and they fit just fine.