I had a relative also with Poindexter, captured, was at St. Louis and Alton and died at Johnson Island.

I believe your relative went to Vicksburg as part of a prisoner exchange. I can not find an old email, but I corresponded with another who had a relative with Poindexter and was at Johnson Island, and I believe his relative went to Vicksburg as a Prisoner of War exchange. Only the healthy people were taken from Johnson Island to Vicksburg.

Your relative may have reinlisted with the 1 and 2nd Missouri CSA brigadess that was captured at Vicksburg. They reformed after being exchanged and fought in Atlanta, Franklin TN and Nashville.

Have you tried the National Archives?