Are Sterno’s safe for indoor use during a winter outage?

Yes and No. This from Sterno FAQ:

Is it safe to use Sterno chafing fuel indoors?

Yes, it is as safe to use indoors as it outdoors. Always use in a well-ventilated area and be aware the can gets hot when the product is lit. As it is an open flame you’ll want to make sure any flammable objects (napkins, drapes, table decorations, etc) are well out of the way. Also, be sure that the can is not in direct contact with a surface that could be damaged from the heat of the can.

Yes, if the area is reasonably well ventilated and away from anything flammable. Sterno is used for chafing dishes at banquets and buffets.

no. not unless you leave a window open near the stove.

No. they give off toxic fumes.