What advice do you have on how I should address a missing diagnosis from an MEB?

Your PEBLO went over ever single claim with you and you were afforded the opportunity to claim any injuries you had. You were counseled by your PEBLO and then signed acknowledgement fo your claims and ratings. These were legal documents that you signed and the opportunity to add anything is gone. The only way to change your discharge is to file for a review with the Army Board of Corrections. They are not likely to review your case unless there was negligence on the part of the board. You were offered several opportunities in the MEB process and acknowledged in writing that you understood your rating.

Now, that only applies if you were medically retired from the Army. If you were separated medically, rated below 30% and then rated at 70% by the VA, all you have to do is resubmit for a higher VA rating. You can still submit for a higher rating as long as you can show the VA that it's service connected. If it wasn't service connected at the time of your separation then it's irrelevant that it wasn't part of your MEB.

Did you apply for VA Caregiver Funds?

At this point, there's nothing that can be done about the medboard not having this diagnosis included.
If you're now getting a % from the VA for this disability then that's really all you're going to get out of it.