I will give you my two cents worth , since I tie 100's of epoxy flies every year for guys.
I never have problems with yellowing.
I never use wood for mixing, I use an old letter opener, it is metal.
I mix all my epoxy on clear plastic. I cut it from 2 liter soda bottles or boxes hooks come in, or lures come in.
I mostly use Z-Poxy but I have used Devcon with no problem.
I store all my flies in the basement .
I put the flies in boxes that don't let light in, I like to use the Whitman Sampler Chocolate boxes.
I do see yellowing when flies are exposed to the sun from guys using clear fly boxes.
I hope this helps.
Tom