Because the Conservatives don/t trust Roberts. The 4 real Conservatives are enough to grant cert and get any case heard before the Supreme Court. They are declining to do so because they don't trust Roberts, not because they don't want to hear the cases.
The 4 don't want to make a bad situation worse.
My guess, which is worth what you are paying for it, is no 2nd Amendment cases until next year. If Biden wins he will do something blatantly unconstitutional regarding gun control. If Trump wins, he will almost certainly have a chance to replace Ginsberg.
I think you're exactly right on everything except the highlighted language. There's only so much he could do with executive orders, and I'm not sure that he even would try. He'd have to have the Senate with him to pass a law, and that's less likely to happen than his getting elected.