Last poll on 538 had biden down by 2. But yeah you are probably right. Still reckon it would have been good to have someone on the ticket to show how much the landscape is changing though. Saying slavery was a necessary evil is something that is likely to encourage folks to vote against him i would say.
I don't know about the quality of polling in Arkansas honestly. I don't think it would be a huge priority in a lot of elections over there. It's been like a +20-30 state for decades? If Trump is actually at serious risk of losing Arkansas, might as well see if Trump can be the 21st century Mondale.
You'd need someone like Bill Clinton to win. Cotton says a lot of dumb shit but as far as I know he's still relatively popular with conservatives. He's no Roy Moore.