I will take a bite. Please excuse me if I get detailed. For backer board I would use a pair of 2 inch wide by about 8 inch long peices of starboard. The restriction is the access to the inner wall surface. Take the mount down make a cardboard templete so the railroad tracks of starboard run up and down, and drill the starboard about 1/16 over sized.
drill a small hole near the top of the wall and feed a light fishing line down to the opening and attach it to a starboard. now you can work to line up your first bolt with both hands free.
the vertical runs of starboard will transfer the load along a taller surface, using a horizonal backing will not offer the same level of support..
L'IL Foot" 2014 Sunseeker 2300
"Harry" the JK toad in the mirror