I think your comparison to a campaign rally gets to the heart of the problem. I agree that at a private event where a political party is choosing who gets in and who doesn’t like a fundraiser or a campaign rally, Trump could hand-pick the signs without 1A problems. BUT, my hypothetical (and the issue at hand) is not a private event, hence the rub.
And the reference to “time and place” restrictions cuts the other way here. Those restrictions deal with placing reasonable limits on things like noise or rallies on streets that can handle the load, etc., they don’t support blanket removal of speech.
