I checked with Cliff England, and you're right, he does not sell any of Peterson's varieties. The handful of nurseries authorized to sell Peterson pawpaws don't sell his scionwood, so if I want to graft Shenandoah onto one of my healthy rootstocks I'll have to buy a grafted plant and then sacrifice it; I'm not interested in wood from non-authorized sources. Since I enjoy grafting and sharing good varieties, the restriction against propagating additional plants is starting to make the Peterson varieties less appealing to me, although it may still be worth getting one if they really are superior. In the meantime, I'll see if Cliff has any Maria's Joy scions available for 2016, and maybe try some of his seeds as well.
For my climate I'm convinced that starting with seeds in the ground is the best way to get a healthy pawpaw tree. I'm guessing the taproot goes straight down to the impermeable caliche layer, and then follows any moisture it can find, which is probably why my seedlings never appeared to get water-stressed from the very beginning.