It's probably grafted on slash because it's a common forestry type seedling that the nursery guy had on hand. Why would you collect an understock? I'm not sure what you mean by top graft. Most common for garden conifers is side veneer graft. Problem with this one is its grafted a foot above the soil line. Which is fine for a garden plant.
Don't air layer it or try to root a cutting. Probably won't work. Whereas anyone with a little skill and experience can get 90-100 percent of grafts to take.
It might be a unique plant. Even if the species brooms with some frequency there aren't any cultivated forms in the trade
Sand Pine is commonly top-grafted to Slash Pine, because Sand Pine is next to impossible to collect. I'm not sure how easily Sand germinate from seed, but Slash germinates anywhere it touches.
It is a top graft, not a side graft. The "top" of the stock is cut horizontally, then a vertical incision is made in the stock, the scion is wedged into the incision. No roots needed on the scion. It's not an uncommon graft at all.
I've seen dozens of these Sand/Slash in person at my local native plant nursery, and they're all done the same way; top grafts. Tops are commonly done with 15gal standard gardenia as well.
We graft our gardenia to nematode resitant root stock, may be the same for sand pine, but unlikely because nematode thrive in sand. Slash pine barks up and grows much quicker than Sand pine, and that's probably the reason its done as a top. You get the barked trunk and then throw the smaller needle on top to make it look like an old pine tree fast.
Sand Pine is a native Florida pine that I am familiar with, unlike yourself. The fact that what the OP has is a dwarf witches broom makes it desirable to separate and grow on its on roots for bonsai, but no other reason. This type of graft makes a terrible union. Totally unappealing and useless for bonsai. On top of how awful the union looks, the bark type is different and noticeably so. In the nursery trade, those things don't matter.
It's not often that they are found in the trade (ungrafted), but they, dwarf Sand Pine, are not unheard and are available for sale on their own roots. This is not an oddity of any sort. Like I said, its very common in the wild. Next time I go to a scrub, I'll photo as many witches broom as possible and I can look through my most recent Plant Finder to see if I can track down a retailer while I'm at home.
To do a successful airlayer on this species, try the methods used with Japanese Black Pine. You can not air layer any old branches or a trunk, but the 1-2 year old candles will layer on JBP. Slash and Sand Pine are both 2 needle pines and should react the same way. These species are to be treated like JBP, not JWP, it is not a 3 or 5 needle pine.
These are not suggestions or assumptions.