You've written a lot of text and cited exactly 0 laws (or case law) to support it. Unless you're a judge, the speculation is nothing more than just that.
Promising a monetary gift in exchange for voting for particular candidates is, both in letter and spirit, illegal. Even if what he's doing is campaigning on some kind of stimulus check, the wording is important, and he tied the promise to a specific election outcome (edit: and I should add that he can't campaign on an election he's not a part of).
He is more than capable of attempting to fulfill that promise with Democrats in office. He chose to tie it to voting for specific candidates.
The electorate is not a person.
You're right. It's every person of voting age in the US. That's a lot of fines and years in prison.