The apples have definitely been encouraging, and the pears have been instructive, so I really can't complain, especially considering how little practical experience I had coming into this. (Like, none.)
Thank you again for posting the link about nutrient deficiencies, it was really extremely helpful. In answer to your question, the fertilizer I used was Espoma Organic Bloom, which is advertised as promoting fruit and flowers. It's a soil drench with a 1-3-1 formulation that also contains 1% calcium, plus a bunch of microbial stuff. Like I said, it was something my wife picked up for use with a potted Meyer Lemon, so I'm sure that there are other options that would have been more appropriate for what I was trying to do. (And I would be happy to learn more about them.) But it was what we had, and it seems to have worked pretty well, at least on the apples.
To Matt: Thanks for the encouragement, and the info about the growth habits of new pear grafts, that's very helpful to know. From what you say, I'm hoping that the root systems will recover and eventually bring the trees along if given some time and a little TLC (without overdoing it). We'll see.
Interestingly, two of the three rootstocks that ended up dying were ones that had some (minimal) growth start above the graft. Hoping to encourage them to push growth into the scion, I pinched out the growth below the graft - but the scion growth never really got going, and in the end the whole thing went kaput.