I too have struggled with bolster alignment. Here is what i do now and it works well.
I drill the holes in the bolsters before cutting or shaping them at all. Then after cutting out i glue them together with a single drop of super glue, using the pin hole as a guide. Then i make them exactly the same profile. It doesnt need to match the tang profile, it is just easier if they start out as identical. When profiling the doubled up thick bolsters, you cant really trust the grinder table is exactly square, so i measure with calipers from the pin hole to the outer edge at various places on both sides, and adjust the table until it is exactly square.
Once the bolsters are matching, but not yet fitted to the tang, you can take a piece of scrap anything (something thin, stiff and can drill a hole in it) and lay it on the tang and drill holes through the scrap where your pin holes are. The scrap piece should be held up tight to the bolster while drilling. Then this scrap piece can be used on either side of the tang to align the bolsters when you go to permanently attach them.
As far as the dovetail, i would use a fence or a clamped block on a disk sander and do the scale ends first. I would lay the scales on a flat surface and slide them to where the angled ends meet and check for daylight, then adjust and repeat until they match up well. The scales need to be flat and even for this to work. Then using the exact same setup on the disk sander i would hit the bolsters to angle them an even amount.
After attaching the bolsters, i would slide the scales up tight and check. If there are any gaps now is your chance to make minor adjustments to the angles on the scales. Slide one scale up tight and glue it in place. When the glue sets, i would drill the pin holes. Then repeat with the other scale.
That should eliminate any gaps you may have.
