You do appear to have RAM problems but those should be unrelated since you have ECC RAM and they are logged as corrected, still need to replace the faulty DIMM(s) ASAP.
Since you did many rebuild attempts and new configs I would suggest first rebooting to clear the logs, then doing a new config with all the old disks, check parity is already valid but run a correcting parity check since a few since errors are expected, only after that's done without any issues try replacing the disks again, using v6.11.1 or v6.11.3, if any unmountable disk comes up at any point during that post new diags.