Q&A for How to Make a Latex Mold

Most craft supply stores, as well as online specialty retailers of casting or model making supplies, will have it.

Hardener or thickener is to make the latex mold stiffer. Latex is very flexible without it. In some cases it can negate the need for a support frame for the mold. In most molds, it's not always necessary, although in some applications it can be helpful.

You can use clay, and be sure to let it dry for a few days before using it.

Yes. Resin and different clays should work as well. Soap and concrete too, I guess (without having tried it).

No. It would destroy the latex mold. However, you could probably pour wax in the mold, and then use that to make a sand mold that would withstand the temperature/heat of the metal. Look up lost wax casting.