I'm basically in the same boat as you, except it's POR and Crystal Palace for the 8:20 ADR.
We are going to ask the concierge at the resort the night before (not calling the front desk, they aren't really the front desk). See if they have any info.
Cabs to MK won't be an option, if the cab can get through, then the bus can get through.
I can think of two alternatives, neither of which are pretty
1. Take a cab to Fort Wilderness (you'll be able to get here)
2. Take the FW bus to the dock
3. Take the boat to MK
Or
1. Take the cab to the "back door" entrance via Floridian Way (if it's open, but I'm not certain it will be)
2. Get dropped off at GF or Poly
3. Take the monorail
The good news: the roads should reopen at 8:00, you'd think the busses to MK would start immediately thereafter.
If that's the case, the busses should start arriving at MK around 8:15 or so. They always allow 10 min for lateness (and probably a bit more with the half marathon transportation issues), so you should be fine.
Again, I'm going to speak to the concierge the day before, and let him/her help me figure out how to get to MK for my ADR. After all, that's their job!