>>However, for some inexplicable reason... Trump remained free to crime...

Not inexplicable, but fallout from Agnew actively breaking the law in the WH (accepting kickbacks and bribes). Part of the 'deal' was that he be allowed to step down in return for lessening of charges - and a Justice Dept internal decision that sitting presidents (meaning Nixon, who was in the middle of the Watergate coverup coming unraveled on him) could not be indicted. There was a need to get Agnew out before the potential chaos of Watergate became public.

OK, so our Constitution says nothing about protecting presidents from indictment (and I think our Founders would have been firmly against it for reasons I won't get into). Also, the Justice Dept is part of the federal government; it has no bearing on state courts.

Why the Feds haven't charged Trump immediately at the end of his term is a bit less explicable. I suspect it's yet another internal Justice Dept thing, mostly around picking up previously dropped investigations - Barr forced the investigation to be halted, replaced personnel in the Eastern District of New York office.

As far as an indictment helping Trump, I expect it to - but mostly among his base, who were going to vote for him anyway. As far as the general voter base, I would hope that it would dissuade them - but we in the US have a history of electing and re-electing convicted politicians, so I'm hesitant to make any prediction.

Just my $0.02