You can only tap a ball bearing on a seat if that seat’s meant to seal with that particular ball bearing. I think if you tried it with a bleed nipple seating, you’d round the seat out, make it worse! As for trying to grind the seat with valve paste … yes, it would work, if the hole wasn’t threaded! As it is, the nipple would bottom out, and you’d have to over tighten to get a face to face grind. Stick with the tape!
As for the half to half seal, try a sheet of 400 wet & dry on a mirror. Don’t go mad. Give it a rub and see where the high spots are.
And, tighten the bolts evenly to spec. on re-assembly!
Inside to out!