shows you the end of the Kitchener grafting and it may help answer your question. It's not as smooth as the one here at KH but see if it helps. You start with an equal number of sts on the two needles so you should end with two sts, one on each needle.
Usually the ballband or the online description will tell you if it's wool or a wool blend and if it's a blend, it'll tell you what's in the blend.
and yes, to get rid of the hole at the thumb gusset, you can pick up an extra stitch and knit it through the back loop or pick up the extra stitch and k2tog.
Congratulations on finishing the first mitten. That's great. Good luck with the next pair.