I've watched a past friend do his best with pawpaws out here in Oregon, but was never impressed enough with their fruit to try (two, they need a pollinator) them myself.
A ---As for the grafting questions ... as no one's responded yet ... here's my opinion- When "Comparing / contrasting / relative success between" rootstocks and fruiting varieties, a lot has to do with when you acquire your "scion" variety. Nurseries appear to "Bud" by preference; as I've watched this done, my guess is they find this method faster and more reliable. Also, it can be done in warm dry weather, while the trees remain in the ground. A team of Budders
lay on a rolling cart with a canvas top for shade, first man makes the "T" cut and inserts a bud; second man wraps (too fast for the eye to follow!) it. Their percentage of "take" is phenomenal. But, this union is possible within a relatively small time frame (a month at best), and you'll need recently acquired scion wood from the parent tree.
"Whip-and-tongue grafting" is more my thing. You can use both dormant rootstock and scion wood, and have several months to acquire both. You can work from a bench ("bench-grafting") or graft directly to an in-ground or potted tree. But size can make much more of a difference with this process. Whereas a Bud can be inserted onto a large diameter stock (of the same age), the whip & tongue is more successful when matched with a similar sized rootstock. I've done many a cleft graft (micro-surgery) on very large stock and comparatively small scion wood, but try to avoid this more brutal
graft if possible. With such size incompatibility between scion and stock you must concentrate on lining up only one
side of the whip & tongue, 'in my eyes' giving it a slightly less chance taking - though it will generally 'take.'
The real difference in "take" is between species
, and that is very likely the main reason for Budding being the most successful commercial process. Like-sized unions give the opportunity to mesh with either side. But when grafting stone fruit; ie cherries, peaches, plums ... dormant bench-grafting is not as successful as budding... I'm not sure why?
"All methods here are reported to be generally OK for pawpaw. Is any better?"
My 'guess' is budding will always be more productive... but, conditions are less forgiving and controllable during the growing season. Personally, with my familiarity with "bench grafting" (and lack of Budding work), I'd prefer the 'dormant' season work and be more than willing to take that chance. I'd used a whip & tongue graft on several persimmons one year with success on all.
B --- "Trimming the taproot (say of well developed seeding or small sapling) to produce a more branched ("fibrous") root system? This method works with many other fruit plants but I've never seen it mentioned for pawpaw..."
I've always assumed a taproot serves an integral purpose with a tree... thus I leave them alone. I also realize keeping that taproot intact is a troublesome yet apparently a critical part of shipping pawpaws from nurseries. You can sever the central leader on a fir tree, and it will 'instinctively' send up another... I wonder if such taproot-trees wouldn't do the same? I've read (years ago) that a pawpaw's taproot must be kept in tact to successfully transplant, and needs to be shipped in a special cone-shaped container with attached soil..? It would be a real shame to sever the root and loose the newly grafted plant due to an experiment...
I'd let it anchor itself and/or seek moisture with that root; my guess: it would simply 'work hard' to establish another one.
Let us know