Well to solve an issue like that-maybe just maybe the editors should have am internal document that is updatable and shared by everyone.
It would say who is off the table to be used elsewhere and why.
Simple-
Medusa and in use for as project
Flint and currently taking pitches for
Explain to fans that is the reason someone is not available. In a project or we are looking at pitches.
So fans of those characters can at least expect something in the future and that might help pre-orders.