Boron nitride can be sprayed
Boron nitride can be sprayed directly on the mold. Just make sure the mold is washed/clean.
In regards to your second question, if you are talking about placing a piece of glass on the convex/rounded side of a mold, if the glass will not “balance” on the mold, you can use kiln furniture to hold the edges of the glass until it begins to slump, and then remove the furniture.
That would not be my first choice, however. I have an aversion to reaching inside a +1100 deg F kiln to retrieve kiln furniture 
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Dana
Jester’s Baubles Fused Glass Designs
