The problem with these questions is that we humans have no ability to discern what God has and is preventing. It’s my opinion that God is constantly intervening in the affairs of human beings to work His will and on occasion we do see glimpses of that.
As far as evil itself is concerned, what is evil exactly? I define it as the works human beings do that are contrary to God’s will and as such cause damage to self and others. I think God “allows” these things to happen for a couple of reasons. The first is because we do have free will. The second is because without an awareness that evil exists, we could never understand what goodness is. And we could never understand how much we are in need to communion with God and forgiveness from Him.
What I think you are saying is that God has no choice but to let man do evil. Is that correct?