Wouldn't a logical conclusion be "we need to reform the UN to actually make it viable in some way" rather than "it's a totally useless shell, don't bother"?
The UN cannot be reformed. Reforms have to get the unanimous approval of the Security Council. The Security Council have wildly varying agendas, but unanimously disapprove of reforms. Do you see the problem here?
What about attempting dialogue with Islamist groups like Al-Qaeda,
We do not negotiate with terrorists.
as well as states like China, India, Russia etc over mutual disarmament.
Will never be accepted. Better to let the gradual growth of peace in the world take it's course than trying something like that.
The USA declares outright neutrality in all conflicts (e.g. with Russia, North Korea, Islamism and so on) and explains its desire to retreat from the world scene to concentrate on its own affairs. Begin drafting lots of treaties that would ensure that America would not interfere in international politics provided other nations don't interfere in theirs.
Our affairs and the world's affairs are the same thing. Globalization is not an ongoing process, it is a thing that has happened. You cannot shut yourself off from the world. Not to mention, the last time the US tried neutral isolationism, the Axis powers had their heyday. It is simply not a good idea.
The USA is the great satan, but if they explained to representatives of the now-truncated Al-Qaeda that they will not interfere in Middle Eastern affairs (including Israeli affairs) I suspect some of that would die down. Make public apologies for the wars in Iraq and Afghanistan, that sort of thing. Even if you consider it morally wrong I'm sure it would get international Islamic terrorism off your backs, it'll certainly get it off ours across the pond.
No, it won't. Radical Islamists do not seek peace. They'll keep fighting the West just because of who we are. That we do not bow to them and believe in their god is reason enough for them to try to kill us.
There's also the small problem that everything you just suggested is political suicide, and that if Israel and the Arab states are left to themselves there is only one outcome, and that is that one of the two are going to genocide the other, possibly through nuclear weapons.