I personally don't think the foam would have been an obstruction to an open casket funeral. Morticians can work wonders, and that seems like something that could be cleaned up. I suspect she was actually shot in the face, and that's why open casket was out of the question.
MOO
I just realized I misread your post and missed the part about that knife. Sorry about that! It's probably time for me to go to bed! But yes your theory certainly is plausible.
If it was a 22 rifle, it sounds like there's little recoil, it's clean entry, but does a lot of damage behind the scenes. Could that mean a closed casket?