Have you tried dumping in Stuff via Pipes? I’m not sure if its already possible, but it definitely works on Molds, and Crucibles do work a little bit like Molds too.
The Smelter is for automatic, same for the Mixer Recipes for certain Metals.
And yes you can store molten Metal indefinitely, heck even Steam can be stored in Pipes indefinitely. Making it become Solid or cold over time would be a ginormous hassle for everyone.