DL
I think the brown & skim coats of lime plaster will be too much for the basement aging area. I'd suggest 2-3 coats of limewash over a clean block surface. Brush the first coat in thoroughly. Adjust the consistency of the wash coats to meet your needs. That should do it!
Your description of the firebox and how it is used leads me to believe that you will be better off with a clay/sand mortar without any straw. Just use a coarse sand aggregate with a fairly even gradation from 1/4" - down to very fine for the strength it provides. No lime or cement will be needed.
