You can drive directly to the Grand Floridian for your meal. Follow pretty much the same directions as for going to Magic Kingdom. At the toll plaza choose one of the lanes near the right (very rightmost for buses only). Tell the attendant you are going to the GF, then drive on and go right at the big fork just ahead. Left at the first light where you can go left. Continue past the Magic Kingdom dropoff lot "cell phone lot" and then past Polynesian, not sure whether this road gently curves to the right or you turn right at the end to get to GF.
I would expect you would make the dinner reservation okay except if Bibbity Bobbity Boutique is running behind schedule. Because of this I would really change one of these reservations rather than leave them this close (under 2-1/2 hours apart).
When you leave GF, you must drive if you were not staying there. You may not leave your car parked at a resort other than yours while you go to a theme park or continue touring elsewhere.
Had your meal been at MK proper, you must park at the Transportation & Ticket Center and use the monorail or ferry. In that case, for travel driving from Downtown Disney, allow an hour rather than 35 minutes.