A one-day road trip from Mobay to Kingston is quite a drive (approx. 8 hours round trip), perhaps even for someone who likes taking in the scenery. If you're okay with being on the road for a longer period of time than you'll probably have to enjoy Kingston itself then your current plan will do. If not, consider spending a night (or two) in Kingston or maybe even spending a few days in Ochi and doing your day trip to Kingston from there (if your travel dates allow for that, of course).
As WANTOTRAV mentioned, the Knutsford Express is another transportation option however you will be bound to the bus schedule. And unless you already know your way around town you would have to hire a tour guide/driver once you get to Kingston anyway.
I know you are seeking two extra travel buddies rather than travel advice but I wasn't sure if you knew how long of a trip this is for someone who is not spending at least one night in Kingston. Good luck with the rest of your trip planning and enjoy your vacation!