Would what he is doing count as tampering? I don't remember the details from his meeting with the candidate, but funding/donating to the campaign isn't likely illegal, as well as using his media resources.
I can imagine Seymour using Perry to somehow disparage the Mayor, in addition.
Ian is too detached from Seymour at this moment (unless he goes as protege in the future) to catch blackmail, but Lena can plan a double-agent and get documents/tape the room/etc to get serious ammunition. However, illegal evidence is not viable in court, and Vermeer is by-the-book type.
Also, regarding Seymour drying their sources of income, both have options. Billy strikes me as a cryptobro, who made good money out of Bitcoin and now spending it on what strikes him fancy. While the game clearly doubts his plan (I'm getting Entertainment 720 vibes), he might have enough dough to make a decent portfolio for Lena. Also, there are hooks about the music tournament.
Ian has Victor White, who is starting his internet publication. If it kicks off, it can help with self-publishing his book and give Ian a decent job.
The idea of the old money (Addingworth and Seymour) unable to influence the Internet (OnlyFans, digital publishing, crypto millionaires) kinda works there - for how wealthy and influential they are, they are competing with literally the whole planet.
He's already told Lena, that he's going to fix the Literary competition so Ian wins. He said Ian will win, if she took the deal with him. The only way he could know that, was if he was bribing the judges. He also suggests that he can make it, so Ian has no chance of winning. So if he's prepared to commit fraud, to fix a Literary competition, the implication is clear that he'll probably do the same thing to fix the city election, because as he states clearly to Lena, he expects to do very well from overthrowing Mayor Vemeer's administration.
He also says he doesn't want to get into politics himself, and would rather be the guy behind the scenes pulling the strings. It's clear Peter Prestley is just going to be his Puppet, who will do only what Seymour tells him to. Seymour is all about control. He wants to control Lena, he wants to control Ian, and most important of all, he wants to control Baluart. A man like that, will do anything to get his way, and we already know that, because of the way he acts towards Lena if she turns down his offer to become her patron.
I think you overestimate the so called freedom of the internet. It's very very easy for the rich and powerful to manipulate the internet, and mold people's opinions. Some people are very gullible and if you bombard them with enough propaganda on the internet, they will start to believe it, because it's on the internet and that means it must be true, right?
I'm not saying there are no options for Ian or Lena, because there definitely are. But only if they follow the path of rejecting Seymour. If Lena doesn't reject him, she never finds out about those options, and Ian never considers rejecting him. Ian doesn't really want to believe that Seymour is bad, because that undermines what he believes is his best route to success. So he's trying really hard to disregard what Emma is saying, even though his gut is telling him, there's something a bit fishy about Seymour. Yes, Victor White might help Ian in the future, but if he doesn't write the review for 'The Fall of Delbaith', and let's Holly do it instead, well he never gets the opportunity at all. So they still both have to follow particular paths, to get these options
As for viable evidence, that isn't really relevant, because Baluart isn't a real place, and is set in a fictional country. And therefore it isn't possible to say what is viable, and what isn't. In any case, all they'd have to do was show him that the evidence existed, and then he could arrange for the police to either raid his premises or put him under surveillance.
And even if they subsequently got no legally permissable evidence, the mere fact that the police were investigating him, would likely thwart his machinations for the election.