First, no they weren't. Nixon had been elected twice already, he couldn't be elected again. That argument of yours doesn't apply.
Second, so what? It was just as wrong not to prosecute Nixon as it would be not to prosecute known crimes of Trump. One could easily argue that not prosecuting Nixon helped lead shape the system which allowed Trump to get away with all his shit.
Let's just take a moment to appreciate the absurdity of this line of argument. You contend that plenty of US Presidents are grossly corrupt - and you use that to support your position that US Presidents should be immune to the legal consequences of corruption? Or to put it another way, because many Presidents are corrupt, you want to make sure that all incoming Presidents will know that they too can be as corrupt as they want and get away scot free? Is that seriously how you think a robust system of government should work?
It's not just a personal payoff, it's the law. It's not just the law, it's imposing some form of standards on those who hold elected or appointed office in the Executive branch. The US claims to be a government of checks and balances. Trump claims that the President has the power to do whatever he wants. Which viewpoint do you think should be proved right?
Irrelevant, the one has nothing to do with the other. Explain how Trump being prosecuted, convicted and not pardoned could possibly have an impact on the timeframe of a vaccine rollout, for example.
You're contradicting yourself multiple time within the same post. Trump is obviously not a lesson to us all, you pointed out already that he got 70 million votes. Again, run a poll right now for greatest ever President and he would win. Run it in 4 years time with Trump still walking free holding rallies and he'd win that too.
Then, it is considered an admission of guilt by people who understand the process. How much faith do you have in the comprehension of reality by hardline Trump supporters?
Finally, since you jst said he wants one, how bad could it be? If a pardon is a demonstration of guilt, a conviction and sentencing is a demonstration of guilt plus an actual punishment. How is guilt alone worse than guilt and consequences?