It does work with them, but strangely I found that while it extended an LED PAR30 nicely for a "new work" recessed light, on a "remodel" recessed light it extended it slightly too far.
Yes, installed new floods and wanted to extend the light bulb closer to the cans housing so the spread of light would increase. Did the job perfectly.
Yes, you can use it on non-IC rated recessed lighting (or any other type of light that needs an extension). It's made of porcelain and is capable of taking hot temperatures. Non IC means you can't have the unit next to any attic insulation or other combustible materials. In my opinion, the extender reduces the heat because it lowers the light bulb which further disperses the heat.
Consult the manufacturer of the housing to find out compatibility.
I used it on the recessed fixture on the underside of the roof of my front porch landing.
Only way you will be able to know if it is just right for your specific purpose is to order it and return it if it does not fit.
Yes, they are stackable.