Tell us about yourself!
Not quite sure if you've finished your project by now or not, though you probably have, but I just finished a hiccup cosplay and I'd have recommended either just going with the black of the plasti dip, or using a matte seal over the black you have used :)
EVA foam is actually slightly an insulator, so yes, it may get a slight bit warm in the armor. Added to this, if you're wearing it for a con you'll probably be overheating within the first several hours
Absolutely fabulous! Hope your brother loves his gift!