When you installed the insulated ceiling did you cut the aligned wall plaster back to the brick (to reduce thermal bridging) and run your vapour control layer down the exposed brickwork. No? I would probably not have done either.
I fear that you are going to need to disturb the edges of your ceiling. Any IWI can then be taken up to the top of the wall and the two insulations overlapped and abutted to remove thermal bridges. The old plaster needs to be removed anyway, unless you believe that there is no dampness that high on the wall.
Changing your proposed insulation and/or wall finish won’t alter the above.
It is fortuitous that you were able to visit the same room in the “same” house in a different time zone. I take it from your description that it is also split be a partition wall.
You may have the answer by copying the past or maybe you could be inspired by the past for the answers of the future. Perhaps a lime based plaster with integral insulation? You could look into Diathonite.