If time is of the essence, and I sense you are trying to squeeze in as much Disney time as you can, a cab may be your best bet. If you don't want to use the cab, I would plan on leaving DTD no less than an hour before you need to get back to WL. If you need to be back at WL at say, 4pm, for DME pickup, you should be leaving DTD no later than 2:45. You need to back at the DME bus stop 15 mins before the stated pickup time.
The buses from DTD to the resorts are all notoriously slow..figure on a 20-30 min wait for the bus to arrive, then 15-20 mins to get to the MK resorts. Now, Murphy's law being what it is..if you get there, planning on a wait of 25 mins, a bus will come along within 5 mins. But, better safe than sorry.