"Trump fears being a "loser", and the likelihood remains that he will depart from the contest at a time of his own choosing and then seek to do other things instead. That's if he remains in good health. Given how few of those present yesterday even in the outside environment of the White House lawn were wearing facemasks, it is quite possible that the trigger for his exit will be his infection with SARScov2."
From your keyboard to God's screen. I actually did have a premonition about him collapsing during his speech, maybe he will at one or another and that will be his ticket out, but I doubt it.
The reason I doubt it is pretty simple, even if the Feds leave him alone as an ex president the states certainly are planning some jail time, even if he can evade convictions he will likely spend the rest of his life out on bail. And I would not be too sure about the FBI and Treasury and other federal agencies leaving him be.
Sure we could not get him convicted in a hyper partisan senate controlled by Moscow Mitch, but not to go after him would be tantamount to a pardon and there simply has to be a price to pay for malfeasance in office, and to discourage other crooks from running just to get presidential immunity in order to avoid prosecution for their many crimes.
So, Trumpster Fire the fat assed orange spy is riding a tiger he dare not get off of, and furthermore he cannot afford to lose. If he seriously believes he can't win, and that is pretty obvious to rational objective people at this point, then his alternative will not be to malinger and think he will walk away scott free, it will be more like declare martial law, suspend habeas corpus, impose censorship, and a curfew that will be shoot first ask questions later. The economic disruption of all this will mean rationing, and what better way to control people than to make every single morsel of food or fuel dependent upon their obedience?
I cannot think the military would go along with it for long, so we might well have a military coup with the goal of reestablishing free and fair democracy, but you know what they say about best intentions. It may well be that Trump has plans to so destabilize the social fabric that he has the justification for this, and it may well be that he finds the possibility of civil war a fine alternative to spending the rest of his life in jail.
That is my speculation on that part of your post; but say he did keel over on stage and could no longer serve or carry on with the campaign. The other speculation your post called for was who would replace him. I think the Trump base would not react well to any republican the wider rank and file GOP would consider acceptable. In other words Trump's exit would expose a deep fissure in the party. For that matter I expect the upcoming election to do that anyway.
Trump lost the popular vote to Clinton 65,844,954 (48.2%) to his 62,979,879, and I doubt he will get much more than 50 million this go around. He bamboozled millions of indy voters last time that now see him as an unmitigated disaster for America and themselves. A lot of republicans are also jumping ship, and many more will just stay home because they can't bring themselves to support him anymore, even as they would never vote for Biden (or any democrat). So he has about 37% of the electorate as a hard core base and they will turn out for him, but that just is nowhere near enough to win. He had record GOP turnout in 2016 along with millions of swing voters, still lost the popular vote, and only won the electoral college by something like 100,000 votes out of 128,824,833 or a margin of 0.0007762% of the voters.
Democrats are fired up at least as much as republicans have ever been. They are going to turn out, and never underestimate the importance of down ballot races in elections like this. As well you can use the polls in those races to figure out what the real mood is in America. Because it might be a coin toss within the margin of error on the one national race that counts, the presidency, but you can see that senators in deep red states are in trouble, Arizona, Colorado, Maine, North Carolina all have sitting republican senators that will flip from GOP to blue, and even states like South Carolina where a black man is dead even with Lindsey Graham, or Iowa where Joni Earnst is in the fight of her life, what we are seeing is that people that are close to the Fat orange one are in trouble.
Not only are you seeing states willing to elect democrats and toss out the orange garbage, those candidates are attracting mor evoters.
It seriously would not surprise me to see Trump get right around the 50 million mark in popular vote. And with population increases in 4 years we could see Biden get well over 72 million. That is what is called in politics a WIPEOUT!