I have never encountered fleas on coyotes but red foxes are usually infested with fleas. We skinned a lot of them but not before getting rid of the fleas. We would spray a Kleenex down with raid drop it in the bottom of a big garbage bag then put the fox (or coyote) in the bag, seal it up overnight and they are gone.
Freezing them off has never worked for me. Shot a fox at - 35 C and left it hang at those temps for 3 days. Frozen solid. Checked the fur and found a flea stuck to a hair that I assumed was dead but as I held it in my hand, and it started to warm up, it started moving
. Not dead. Into the bag with the raid Kleenex it went.
Good luck Skinning