Pouring concrete slab against 2 block walls and an asphalt driveway
be sufficient as screeding surface to get the slope right and an expansion joint?


No. And neither would the stuff I normally use; Fiber Expansion Joint, 1/2" x 3".



Tapcon a screed guide board to A (and/or B), at a height that will allow a board that's screwed to the top of the actual scredding board, to ride on it. That's how you screed something with only access to one side. And it's what you put the plank on to reach the corner to trowel it. 

Ideally the guide board is an inch or so above the finished surface, otherwise you can't get a trowel in there. E.g., use a 2x4 for the guide and a 2x6 for the screed. Screw a scrap 2x4 to the top of the 2x6 so that it sticks out 1.5". Have fun jiggling it back and forth 3/4" at a time.


What should I use to frame side D


Frame as usual with 2x, well buttressed. Remove later, add expansion joint, and then fill/sculpt by hand.


Once the pad is poured...


You're SoL. The expansion material goes in before you pour. You cut them to length, put them in, and they fall over. Start filling and then stand them back up.