I would say your chances of getting an FO slot are very good. I just can't say for sure how they would be specifically for BFI. Basically because I am not based there and don't know what their FO situation is.

You say you won't be available until October. How much flying do you think you might be doing between now and then?

Also, for part 135 your X-country time isn't restricted to just your over 50 nm flights. Any time you take off at one airport and land at another is considered cross country for the purposes of the 500 hours needed for a 135 captain.