Sandy, I can uderstand how you came to your idea, but have to reiterate that I think it isn't ideal. Again, I would put a shrub, or several shrubs there in the corner, because a tree is going to grow into/over the sidewalk, blocking it with branches, and gnerally getting in the way. If it's a BIG tree, then it may well also grow long enough branches that it needs to be trimmed away from the side of the garage. A shrub in that location will maybe get wide enough to run up against the sidewalk or possibly the driveway, but the right one won't. With a shrub, you can know beforehand about how wide it will get, but most trees aren't so obliging - there are exceptions, but... If you want an evergreen shrub, do you want a tall one (4-8') or will a couple of shorter ones (2-5') do, or do you want both heights? Either, coupled with an tallish evergreen tree by itself in the middle of the lawn, would give enough visual mass to balance the addition. Me, I think I might go for a mix, maybe a small or medium evergreen to provide winter mass, and something like a butterfly bush to give summer interest. You can keep Buddleias trimmed to the size you want, and in fact, should trim them in summer after a bloom flush to get better reblooming. If you don't want to cut a shrub back hard in the spring, and a couple of times in the summer, then a crepe myrtle could be very nice. Since you can chose cultivars that range in height from 3' to 40', and colors from white, red, purple and pink, you have a wide variety to select from. A rhododendron or a camellia would also give you an evergreen, with flowers in winter/spring, with azaleas at the base for lower fill-in. Selecting for height and color could give a very nice effect with either or both. The camellia can be trimmed, if you really want a formal effect at the corner. For a taller evergreen tree, out in the lawn, I might go with a conifer - NOT a Leyland or something that will get 40-60' - but an interesting one with a good form or maybe a variegated or color selection. I would LOVE to have a weeping Atlas or Alaskan blue cedar growing in my lawn! You could also go with another magnolia, or with a holly - some of them can get pretty tall. There is certainly enough room for a tree in the lawn, unless, of course, you like the expanse of it and don't WANT another tree out there....

If you are back East this is one of those kinds that has been the cause of multiple posts wondering what was going on with their tree. Other than that, it produces a spreading top of greater width than you have indicated - when fully developed. Specimens have been measured with average crown spreads more than double 20'-25'. Visually the tree is a sort of cattleya orchid, with a sensational flowering contrasting with a utilitarian foliage appearance. Fall color is more than yellow but not spectacular. A modern, disease-resistant flowering crabapple cultivar might be a better bet for you. Or maybe a service berry with good fall color, as long as pests and diseases in your area do not make these a gamble.

Maybe next year, if continuing to overwinter. Suppose it might be known to lose flower buds but not vegetative buds in winters outside of its tolerance - somebody from your region may have noticed. Otherwise, if tips not dying back presumably it is overwintering OK, so far.

It's quite hard to give general advice about zone-pushing (or zone denial) without more specifics about your location. Where are you and how long is your growing season? Are you in an exposed location? Very generally, it seems that you're pretty marginal for all of your listed species. Flowering dogwood survives as far north as Ottawa (zone 5-ish), but it's very likely that low temps will kill off most or all of the bracts (no showy 'flowers'). Redbud is equally iffy. There are, however, a number of lovely flowering trees that will survive (and maybe thrive) in zone 4. Black locust is really underplanted, and it puts on an amazing display every June. American linden (and many of its hybrids) has an amazing fragrance, and is well worth a look. And, while I'm never a fan of invasive foreigners, I have to admit that I love the smell of Amur maple when in flower.