No, Flightpath is on the Airport grounds but is not the one attached to the Airport building by the overhead walkway which I think is the one you are referrring to. I could be wrong here, but I think this is one is not pre-bookable and can only be paid for at the machines on site. No idea on costs though but it will be the most expensive option because it is the most convenient.
Check-in for domestic flights depends on your airline. Most open 2 hours prior to departure and close 40 minutes prior to departure so I would not chance 30 minutes unless you are travelling to a "diddy" Airport, such as Campbeltown
when you might be okay. Alternatively you can use online check-in with most airlines which usually opens 24 hours prior to departure and this will allow you more slack if your time is tight.