The old kiln wash needs to be removed before applying new. If not, you’ll see flaking as you report. That may be what has happened.
Yes, once you use BN, the kiln wash will likely not “stick” unless the piece is heavily sanded.
Rough edges could occur for a variety of reasons. You can use a scythe stone or diamond hand pad to smooth it out.
Jester’s Baubles Fused Glass Designs