Yes aluminum will melt using this method.

Concerns about toxic lead are somewhat over blown. Unless you do this day in day out with your nose over the crucible you’re not likely to get any ill effects. Though no lead vapor is better than a miniscule amount.

As far as frying the magnetron goes, most of the radio waves are absorbed by the SiC refractory placed inside the microwave, but if you’re that concerned a) toss a glass of water in there to catch the extras b)cool the magnetron using a bigger (active)heat sink.

You definately can melt silver in a microwave, There are a few custom jewelry makers that use ceramic molds to do so. The frequency of operation doesn’t have much to do with the ability to melt a given metal due to the fact that they aren’t heating the metal directly. They all go into the crucible/refractory which in turn heats the metal. The biggest hold up is going to be the microwave wattage and the ability to keep the magnetron inside of it’s safe working temperature. You’re probably not going to be able to melt more than a few ounces of silver but it’ll still melt.