In regards to the cleaning, I would use acetone or something like that to remove mold release from the inside. As for the fin, if the thread I loose in the fin you can either go with a large screw and just make the hole bigger on the jet or you can add some hysol to the screw and secure it. Just make sure the fin has a block of wood inside for the screw to attach to. Welcome to jet building

What bulkheads are you missing?

Also for the future, the ceramic paint is heavy and generally you only need it around the aft part of the turbine and bell mouth area. Lots of room between the turbine and skin in this particular model, other jets that have a tight fit turbine may require more area. I personally wait till the turbine test runs and check for hotspots then add heat protection. The wires in the tail all get heat protection during the build, I just hold the turbine area till its ran.