Congratulations on getting into flying! You'll love it!
I think right now there's too many things that can change between now and when you'd be eligible to work for Horizon, that some of those specifics aren't going to be helpful. To my knowledge SEA is fairly junior for Skywest FOs, but might be senior as a captain. Horizon may not place is you Seattle right away, but you could possibly bid into it within a few months of working. Knock out some ratings first and then work on applying into a cadet type program if you're still interested. Good luck!