can you use a 10 speed chainset with 11 speed group set
it won't be ideal but it should work. I'm running an 8spd chainset with a 5700 10spd group. works great
Yes you can absolutely. There is no difference which will alter the way it works. You could use 8,9,10 chainset with any 11 speed derailleur and cassette
I've used 9 speed chainrings on a 11 speed setup with 10 speed chain. All worked great.
Absolutely no problem whatsoever, as above you can use almost anything to hand, in fact I've run a 1970s Stronglight with 11 speed chain just to see if it would be ok.