Both sides have Gerrymandered, it's true, but (currently) the map is heavily tilted towards Republicans. They won overwhelmingly in 2010 and got to redraw the maps (Project Redmap). You can see a recent Supreme Court case on this where they acknowledged that it's all bullshit but "the legislatures could fix it if they wanted to" (hint, they don't want to - they got elected with these maps).
Current gerrymandering. I'll clarify that this is now
explicitly legal; it just also runs completely afoul of the ideals of a functioning democracy (or republic if you want to be super pedantic).
Mail in ballots have nothing to do with gerrymandering. I'd also be interested to see if you have
any evidence you can cite that show Democratic tampering with mail in ballots. All I know is that a GOP operative was caught finagling absentee ballots in NC in the 2018 primaries.
There are some people who would split ticket vote but they're a minority, I assure you. And there are more people who hate Trump (55% disapprove) than generally vote Democrat so I find it extremely hard to believe that there'd be enough to vote for him and a Democratic representative.