a) It's not slander if it's true, Deus wouldn't have even the slightest grounds to work on. Also the other Senator doesn't necessarily have a smaller PR team.
It hardly matters if its actually true. What matters is what can be proven in a extremely expensive court case, that Deus is likely to win.

b) Well it depends how hostile people are towards Deus. I mean, realistically speaking, there should be people flat out calling for his head at this point. But the author is very gentle towards his characters in regards to people getting upset.
What? No it dont have anything to do with how hostile people are towards Deus. There could be mobs howling in the street for his head, but it would not affect if Sued Soap Special was quietly holding fund raisers for someone, that some of Deus's friends just happend to attend.

Eeeeehhh... hard to answer that without getting too far into real-world territory, but suffice to say that history contains a remarkable number of examples of utterly psychotic behaviour being excused and covered up because the psychopath in question had money and connections, even starting (or at least facilitating) wars- e.g. William Randolph "You furnish the pictures, I'll furnish the war." Hearst. Even if Deus is getting away with maybe more than is entirely believable... eh. Sydney'd probably have been on KP for the entire duration of the comic with the way she acts if it was a 'realistic' storyline.
Yeah. Deus is a popular handsome Billionaire. Basically American Nobility/Royalty.
There really isnt anything unusual about what he has gotten away with personally, just by going on RL examples.
And thats before we factor in supernatural intelligence and manipulation skills.

The same with the war in Afrika. Its more plausible he gets a medal for creating jobs for the gun industry, than sanctions.
Especially since he isnt running the country. He is just an "advisor".

So no. I cant really think of anything he has gotten away with due to plot armor.