You are right if Trump wins many activists and grass root organisations will not take it well but let us not forget Trump has his own supporter base including evangelicals and the right-wing so this Presidential election will get messy.
Isn't that how it should be, Biden's supporters, the BLM and those who think Trump has had a bad four years as POTUS wouldn't celebrate if Trump wins, there could be maybe few riots here and there from members of the BLM, but that will be all, nothing significant, the U.S. is a first world nation and I think if there's any case of post election violence, it will be extinguished asap.
Many of the postal votes will be counted after election day and both camps will definitely contest the result either through the media or maybe in the Courts
When politicians go to court after losing an election, it usually doesn't change the result at the end of the court hearing, sometimes I wonder why they still make that move of going to court. In some third world countries, post election cases can last over four years (adjournment upon adjournment) by then the winners tenure is already up, lol, it's a waste of time imo.
It's definitely possible that what you're describing is what happens.
But if it's even a somewhat close election and Biden is declared the winner, it's entirely possible Trump just says 'no, he cheated, I won'. or 'we need a do over, here's an executive order now you have to do it' or whatever.
And then what.
Trump still has a couple months with all the powers of the president. The entire Military reports to him, he's already demonstrated how easy it is to pretend like congress doesn't exist and manipulate the media, there are millions of Americans that actually believe the things he says - no matter how irrational or obviously untrue they are - and millions more that think it's funny when people get pissed that he lies so much. That's a shitload of power.
As soon as we don't know who the president is, (half the country considers Biden President, the other half thinks it's Trump), we're no longer a first world country or the
United States.