My reasoning is based only on what I have seen and heard. I'm pretty sure that I have not claimed any first hand knowledge. All I know is that there are bits and pieces in front of me and I want to assemble them into a larger picture just like a jigsaw puzzle.

Here are a few things dancing around in my mostly empty skull:
Did Lewis underperform or did the car let him down? We can't know if Russel would have won the race had Mercedes not screwed it up for him.
Earlier in the season Perez was forced out of 2 races so why the rush to get Lewis back in the car for the very last race even though they had already won everything possible.
I doubt that Lewis' contract would have any penalties regarding illness. But F1 must certainly have restrictions regarding drivers, or anyone, that bring their symptoms to the track. Why was Lewis exempt?

If this were a movie (and in my head it IS a movie) then I think it might go like this:

A demanding driver has reached the end of his contract and is negotiating to renew it for next year and beyond. Suddenly he is stricken by the latest virus and is forced to sit out a race or two. But the team must move on to the next race and are forced to replace the driver until he is well enough to return and is free of symptoms. Watching from his sickbed, the driver is horrified to see that the replacement is not only winning but putting distance on the rest of the field.
This is terrible! His insecurity is getting the best of him. He wonders, "What if that guy is better than me?". "And what about my contract? Am I in a worse position now? I wish this replacement guy was having a terrible day so people could see for themselves just how valuable I am? I've got to get back in the car next week".

He makes a phone call. "Hello, Toto! I'm coming back next weekend!". "We think you need to stay home and rest", they say. "After all we have nothing to gain", . "Why is this so hard?", says the driver to himself. "Why it was just last year I was able to force the team to let me air my political views".

The driver shows up with symptoms and does poorly. Now what?

There are 3 stories here:
Why was Lewis allowed on the grid in the first place. Having a negative test does not mean that you can't pass along your symptoms. How did he get away with that?

I think Lewis demanded to come back and Mercedes caved in. My respect for Mercedes has taken a big hit for their lack of courage in this case and willingness to give Lewis a platform for his political views. No other driver would be permitted to exhibit that sort of behavior.

Both Lewis and Mercedes should be ashamed of their treatment of George Russel. He drove a magnificent race and should have been rewarded after the team ruined it for him. Instead, they threw him under the bus. Just pathetic!

I should also mention that Williams now has an opportunity to do a great thing for George by releasing him from his contract in such a way as to save his seat but be given the chance to shop his skills to a better team. It would be a great gesture.