Off the top of my head, I think the model fire code that shows up in some load data pamphlets has a maximum of 25 lbs smokeless and maybe 5 lbs black. Not much for some people. 50 lbs is the most the black powder distributors will send in a single order; and that as two 25 lb cases. But if you were to go strictly by that fire code, you would have to store it in a separate magazine.
The recommendation for powder storage is in a wooden box. Wood is pretty good insulation and slows heat transfer even while charring. Also if it eventually burns through and the powder goes, it does not contribute shrapnel like a metal container.
A lot of huhu. I have on my new house's mantelpiece a scorched Goex can and a blackened whisky bottle recovered after the fire that totaled the old house. The contents are fine. Were fine, just fine in the case of the Jameson's. I've still got most of the FFFg, though.
I got no static from the insurance company, largely because NOTHING HAPPENED. I have not found where a single round popped in a melted box or bag, and the powder cans just sat there.