for query 1, that is a tough one to answer, but can wager that it is less likely for roots to damage pipes if the soil the trees are growing on is constantly moist, as there’s no physiologic need for roots to grow thick into tight spaces if the vast expanse of mother earth is moist throughout. Tropism only gets heightened where the desirable stimulus is isolated/situated, so in your case, the stimulus(that is moisture) is pretty much everywhere, so no need for roots to converge, network, and enlarge at plumbing systems. Of course there is always a risk, and not just with jujubes but with other trees, especially larger and fast-growing species. Jujus are not particularly large trees. They will grow relatively fast the first five years but inclined to slow down thereafter. The ~20 yr old jujube tree have been watching at UNLV has not even attained 10" caliper of main trunk thickness and not much taller than 15 feet. Here in hot and dry las vegas, we’re growing some ~10 yr old juju trees quite close to our house(tallest is ~ 15 ft tall, and all with calipers of less than 5 " at the thickest portion of main trunk) and will definitely update once we start having plumbing issues. Important to note that certain rootstock(often the wild-type spinosa’s used by many nurseries) can sucker prodigiously, sometimes more than 20 feet away from main trunk, and this can be a problem if you let each sucker grow into trees. Consistent decapitation of suckers will inhibit the suckers’ buttress roots from thickening. Btw, the tendency to sucker varies, as some nurseries use random seed, so there’s no way of predicting if the rootstock you’ll be getting will be invasive. *probably good to buy cultivars on their own roots, which has been a recent craze, as posted at @mamuang 's thread. HJ would be a good candidate due to its small-ish growth and skinny trunks, so even if suckers grow into trees, the roots beneath shouldn’t get too thick to lift the concrete slab off of one’s driveway. Besides, an hj on its own roots suckering would be considered by many here as extremely desirable instead of a pest!
have to mention that @BobVance also has a relatively old So that seems to be dwarfish(just recently determined that there’s at least two cultivars of So being sold by nurseries).
for query 2, we plant them at ~3 feet apart here in vegas, but clearly a different story in new york where you’d want your trees’ branches to be getting maximum exposure to sunlight as @fruitnut mentioned.
for query 3, also agree with @fruitnut that gainesville will likely be more conducive to production, although also have to say that @mamuang and @bobvance(northeast usa), @tonyOmahaz5(nebraska) and even @Chills in michigan have gotten theirs to fruit, and some have posted photos of incredible production-- for their respective locations.
and speaking of gainesville and jujus on own roots, would be wrong for me not to volun-tell @Livinginawe to give you tips, since he lives there, and has been doing amazing stuff with his juju trees.