Regarding Culber being on that supposed memorial wall, it should be noted he's listed there as a full Commander, a rank he was not promoted to until arriving in the 32nd century. If that is indeed a memorial wall, the implication would be that he's now dead.
The existence of Rear Admiral Lower Half has never been canonical in Star Trek, and given modern shows do feature Commodore as a rank, that is what Starfleet uses.Commodore would be RDML (Rear Admiral -- Lower Half).