The cases against him are obvious hokum, but since when has that mattered? If the government wants to destroy you, it will succeed. Now, if you're lucky, rich and young - you might be able to spend 20 years trying to clear you name, and then (and even that has only occurred in non-political cases) belatedly succeed. Trump does not have that sort of time - he's old and morbidly obese.
There is a second cope that people will see through the fact that these cases are hokum - as little as that would help him. While that is not impossible (and Trump has defied political gravity many times before), there is an equal - if not greater - possibility that enough people will be swayed by the indictments against him. "I'm voting for the guy who doesn't have 79 indictments against him" is a reasonable statement if you live in a country with a rule of law and isonomia, which many people imagine they live in.
There is a silver lining. The first generation of reformer never succeeds - think the Gracchi brothers - and often comes to grief. The regime has here put itself in a lose-lose position. Imagine if they get what they want, and Trump is convicted and is sent to prison until he dies. They still lose, because they lose the fig-leaf of republican legitimacy, and had to behave in openly banana republican ways - permanently alienating the 30-40% of die-hard Trump supporters. It does not matter, you say, but it does - that's why both the regime and even openly autocratic states try to retain legitimacy for their power and their elections.
If they lose, and Trump is re-elected, it's more obvious why they will lose. This is a man they could have easily won over with a little bit of flattery and just by not being obnoxious (cf. Lindsey Graham and Mike Pompeo). Instead, they have made him lust for their blood, and God help them if he ever is re-elected. Here too they are in a lose-lose - either he will use the powers of the presidency to crush them, or the impotence of the presidency itself will be exposed. Both rather undesirable situations for the regime. Which is why I think they've gone to such desperate measures, and may even resort to assassination as happened to the Gracchi - and they'd get the same result in sowing the seeds of their own destruction just as the Optimates did.
None of this is to suggest "giving up", because I think everyone has an obligation to fight for what is right regardless of the circumstances.
Even as short sighted and incompetent as they are, they're afraid to kill him. The Trump train might not be running as strongly as it was (he really did have a disappointing first term in office), and the right might have taken a lot of shit until now, but there is a non-zero high chance that assassinating their leader would ignite a huge powder keg.
But the powers that be are morons. Cunning manipulators and immoral liars, but complete idiots. Maybe they underestimate the powder keg, or in fact think they want to take advantage of it. So your scenario is possible.
Personally I think he gets back in office and does nothing useful. His base cheers and declares victory, Trump bloviates, we make some small victories, nothing really gets better. I do think his heart is in the right place (to the extent it isn't dwarfed by his ego and desire to be popular) but I don't think he wants to get his hands dirty with anything real.
We can survive that if we don't fall into complacency. The real change has to come from us. And it already is.
Like you, I think they'd rather avoid that. But assuming for a moment that the presidency holds real power, then they'd rather assassinate him than allow him to ascend to it while vowing "retribution".
It might also be that they want to ignite a huge power keg in order to justify more repression.
Ah yes, you already thought of that.
Unfortunately, I think you are correct.
Just imagine the defeat populism would have suffered if they hadn't scored the dozens of own goals. No Russia hoax. None of this criminalization. Just treat him like a perfect gentleman, then Covid strikes, sabotages the economy and gets him down.
Instead, they had to act like total psychos.
It's true, also, that a cornered animal is desperate. If Trump does represent a real threat to them - and there may be some hope of that, since they're trying so hard to stop him, although I think that's more about the populism he represents - they might decide they have nothing left to lose.
I resent them for that more than I express, but since it revealed their true selves, I'm glad they did. At least we know what we're fighting. Better yet, a significant amount of people are waking up to their ways. Their hubris is their downfall. I hope.