If you have a low graft the branches are a bit more likely to touch the ground than a high graft. If you study the trunk closely you can see where a scar formed where the root stock was grafted with the 'Ever Red' scion.

A low graft might not exceed 5 feet for quite awhile whereas a high grafted tree might not take so long.

I don't suggest pruning it by cutting the main leader that makes the plant grow upward. Unless highly skilled at pruning ornamental trees, more often than not, it just won't look right. Thinning cuts can be made to braches that are growing inward toward the trunk or for branches that are crossing. The best time to prune is the dead of winter before the sap rises in the spring. You can remove dead wood any time of the year.

Japanese Maples roots are not invasive. They do not send deep tap roots and are generally shallow without breaking the ground and most are fine roots. They don't crack cement like the other larger maple cousins.

You say you want to plant it about 3 feet from the driveway. The total spread one day might be over 6 feet, its hard to know so I am thinking 4-6 feet might be better in the long run. The maple might have better water uptake when it matures if it has a bit more room. The maple won't get much water from any roots that go under the driveway and might cause problems later.

Your Red Select if probably the same tree as 'Inaba Shidare' from what I understand. Good companion plants might include black mondo grass,yellow or dark green ground covers with good layer of mulch. I like the japanese forest grasses. Search for Hakonechloa macra, one of the shorter varieties appropriate for the sun and zone you are in. I guess companion plants are personal preference, hosta may work fine, I have no idea. I would steer away from anything that might steal all its water or has aggressive roots.

I hope some of this helped. Good luck.

Although frequently mislabeled, 'Ever Red' is a distinct cultivar, noted for the very fine silvery-gray hair-like quality of the newly emerging foliage. Most dissectums are sold with low grafts and will form low, mounding shrubs that will seldom ever achieve 5' in height. Generally, only those with leaders staked and trained vertically will produce the maximum height and then only after many years.

I agree with matt that the canopy spread can be considerable, more so with those that have low grafts and are allowed to grow in a mushroom shaped mound, and allowing for the potential of that spread without interference is a good idea. As to companion plantings, all of the above are good choices and there are many others. Just avoid those plants that require frequent dividing that will disturb the tree's root system - not a good idea :-)

Good point about not planting things that need dividing. I was typing after midnight and didn't even occur to me. From what I remember of Hostas, they need dividing. I such a poor memory though. :)

No, hostas are long-lived tight clumpers like peonies that would just as soon be left alone. In fact it has been recommended that a pie-shaped wedge be cut out of hostas when attempting to propagate them by division rather than popping up the whole clump and disturbing it.

Dwarf hostas would make a good companion plant. Other good companions include Dwarf Iris (I. cristata), Creeping Jenny, Ajuga, various Sedums. All of these plants have roots that wouldn't compete too much with JMs. Dwarf irises are especially nice because their roots (corms) crawl near the surface of the ground. They also bloom nicely in the spring and have very attractive foliage that contrasts nicely with JMs. However, they are best of a shady to part-shade location, not full sun or hot afternoon sun.