US SEN 2020: Which GOP Senate seats are the most vulnerable?

[Kamala]

In my dream, Jason Frerichs wins the 2018 open House seat in South Dakota and then turns it around and unseats Rounds in 2020. Not gonna happen though .

I think you've got a good list. If I had to rank them in order of competitiveness:
1. Gardner
2. Tillis
3. Collins (Open)
4. Perdue (Open)
5. Perdue (Reelection)
6. Ernst
7. Sullivan
8. Daines
...
9. Capito
10. Collins (Reelection)
...
11. Cornyn (Open)
12. Roberts (Open)
13. Cornyn (Reelection)
14. Roberts (Reelection)

[Kamala]

What would happen in the unlikely scenario Moore Capito retires?

Republican hold either way, West Virginia is too Republican to support a non incumbent Democrat in a Senate race.

What about Jim Justice? WV voted for a non-incumbent Democratic governor in 2016, where Trump won every county. Would Justice lose to a weak candidate like John Raese?

I mean I doubt Raese would win the nomination, since it's likely that, out of Jenkins and Morrisey, whichever didn't get the nom in 2018 would likely be a shoe-in for 2020.