Minniesgal is right - it may be impossible to get an afternoon rest in the room at the resort. With only one night your room may not be ready even at the 3:00 pm check in time. We have been as late as 6:00 pm getting into our room (although getting into the room around check in time is more common). You might be lucky and get into the room early in the day but you need to know that it is always possible that you also might not.
There are no boats that will take a wheelchair from the monorail resorts to the MK, unless you stay at the Poly and walk to the TTC and take the ferry boat from there.
The monorail will take you from either of the MK resorts to the MK, as long as it is running. If it is not running, then there is a bus provided or the small boat (NOT wheelchair accessible). That narrows your choice down to the CR where you can walk to the MK even if the monorail is not running OR the Poly where you can walk to the TTC and take the ferry.
I agree with PP's that the CR is the best option.
ETA: from your other post, I note that you are considering FW cabins. That might actually work better for you. With 2 days there you would be able to go back for an afternoon nap. Also, you can park by the cabin and drive to the MK parking lot (as you suggested). From there you can take the ferry or the monorail (if running) to the MK. OR if your cabin is near the boat dock at FW, you can walk to the boat and take the boat from FW to the MK. Many of the boats running from the FW to the MK are wheelchair accessible.