No knowing how that brew will behave, if the resin is completely moisture proof then ok, but it isn't. I wouldn't want to use it.
No idea what your drywall stuff is, would probably be gypsum here, not much of a thermal conductor but likely harmless.
Marble flour works perfectly well, I can't really see that silica sand wouldn't be ok. If you have that magnetite magnetic sand then I would avoid that but even then I doubt that it would cause much trouble.
I am sure many things will work but unless you use something that has been tried and found to work then you must take a chance.
Here you go to the shop for baby powder ( its talc here and nearly every shop keeps it).
Flux