Hi Snug, bit late catching up with some threads and tripped over this one.
Bummer about the neck heel joint fit and at first it looked like a 4 string body with a 5 string neck. Good to see it was just 1mm out.
For the fretboard you may get away with just using Dingo Wax or similar if using flatwounds as Double Bass boards are basically left unfinished. If using round wound strings they tend to bite and dig in and that is where a harder surface may be required to protect the softish rosewood. CA glue is highly toxic and the fumes are wicked plus it is not so easy to get a smooth finish as I found out when converting a fretted Bass.
My previous Yamaha fretless had an untreated ebony fret board and I was using round wound strings with no ill effect but would recommend Flat Wounds and Wax as the cheaper and easier way to go.