I have three tree questions:

Question 1 Did the March tornadoes have anything to do with the many trees that never leafed out (and now appear to be dead) or only partially leafed out this year?

Question 2 I noticed that many of the trees that fell during the storms had hollow trunks. Is there any way to tell if a tree is hollow and needs to be cut down while the tree still appears to be healthy?

Question 3 A few weeks ago we had another wind storm that knocked down more trees and I noticed that some of those trees' trunks were filled with some sort of black gunk that looked like mold. What is that stuff?

If the trees were badly damaged by the storm, then they have prevented the leafing out but I suspect the trees were already in trouble and the storm helped them to finish dying. As for the wood rot, the trees need to be core bored by a reputable arborist to determine if they are rotting. some trees are notorious for having wood rot. Most fast growing trees like populars, willows, boxelders, and silver maple. In addition, oaks in the red oak group tend to get wood rot. The older these trees get, the more likely they have wood rot.

Not sure if the black gunk is due to a decay fungus or if you are talking about wetwood. Wood rot softens the wood before hollowing it. With wetwood, the wood stays hard but the internal wood discolors often to a dark brown and if it is wet enough, the water in the wood foams/bubbles out of the core wood. If not one of these two problems, I would need to see either good pictures of the problem or a sample of the affected wood.