3 tree questions

Question 1, the half tree. No idea of species. It's hollow, sometimes spouts mushrooms out of the hole, etc, does leaving it to do its thing do any harm? There are no other trees of the same species anywhere nearby that I have noticed.

Question 2, the Maple, suggestions on sequence to remove the extra trunks and prevent rotten crotches?

Question 3 (not photoed) I have a red oak in my front lawn that is relatively young and healthy (12-14 inch diameter). There is confirmed oak wilt in my county (but it is not yet pervasive). What steps should I take, including when should it be pruned, to prevent exposing it to oak wilt as long as possible (I assume it is inevitible here in IL).