I've baked\boiled soil-less mediums to kill off possible hitchhikers before, but not actual soil. I don't know how hot you could go before killing off bennies\etc. Would have to be way hotter than the natural reactions that take place when the soil is breaking down in the first place..
It's reasons like this that I prefer to grow hydro. Don't get me wrong, you can still get infested, but at least you know your not supplying the constant migrational source of dormant gnat/mite eggs just waiting to hatch..
My only real suggestion for the gnats would be to wrap the soil in a fine mesh fabric, like panty hose liner, and let it sit around for a few weeks or more. Or, wrap the entire pots with the plant in it, all the way around and rubber band around the stalk. You can still water and get drainage, but the adult flyers can't go in or out and will die off.
About the mites though.. yea. I would say store the soil (everything you own actually, all your gear) in a sealed off tent of sorts, with a bunch of hot shot no pest strips. Let them permeate everything with the dichlorvos fumes in the strips. Keep churning the soil once a day to make sure it's all been fumigated through.
I would'nt mess around with any other alternatives\products unless you like waging a long war that cost time and money.
Kill it all off in one shot and take a break until its done.
