Maybe a 'Shishigashira' Japanese maple. Japanese maples can have a problem with blighting off, if you plant infected stock or plant in infected soil. Otherwise, this variety in particular I think would be highly suitable.

I'm sorry to hear of your need to find a tree, but I want to suggest another wonderful one: the clerodendrum trichotomum (peanut butter tree). Three-season visual interest, wonderful fragrant flowers that attract butterflies and hummingbirds, berries that birds love...and street-tree-tough. Withstands even verticillium wilt, I think, which jap. maples don't (I know from sad experience). It can handle that amount of light.

We planted a memorial tree for my father-in-law a few years ago, and have been pleased with the Blireana Plum (Prunus x blireiana). It has lovely reddish leaves, can be kept to 15 feet (naturally goes 15-20 ft), and has really pretty and early pink flowers (and a whole lot of them). Here is a pic I took last week of one of the early blooms.

Clerodendrum may sucker madly, of course. Some Prunus x blireiana up here have been taken out by cherry bark tortrix. You will not find any tree (or large shrub) that will stand there like a statue, of course, but some will be closer to your ideal than others.

well yes, it suckers and seeds, but you can give those away or pull them. The beauties (visual and olfactory) and hardiness way overbalance any suckering, IMO. Other people's opinions may differ :)

Many Japanese garden favorites, including Japanese maples will tend to be suitable as they fit into the requirements of a formal garden, where the plants are often used as specimens or objects (and tend to be woody). If I was going to try a Prunus x blireiana type specimen I would use its much more appealing parent P. mume and skip the P. cerasifera 'Pissardii' part of the characteristics of the hybrid, including inferior floral aroma and purplish leaves.

Since both may sometimes be hit badly by bugs or blights I wouldn't recommend either for this situation.

Pretty hard to keep a purple leafed plum to 15', not to mention deal with the problems these ultimately generate. And they are extremely common, being the first choice of the cut-rate landscaping prevalent in most new developments. Japanese maples are always a good choice - many to choose from that will stay within your height restrictions and all offer foliage attraction in several seasons.

Another consideration is something like a witch hazel. Slow growing, seldom to exceed 15' (except possibly after many years) and with a colorful, early - like now! - bloom and often great fall color. Would be ideal for the location you describe and something a bit beyond the typical choices.

While most purpleleaf plums do grow taller, Prunus x blireiana is half P. mume and has a similar low habit.

"'Blireiana' is a natural dwarf tree, rarely seen even 15' tall, its warty little trunk practically never a foot thick. In full bloom it's like an enchanting Easter miracle, cute and irresistible--a filthy and impoverished street urchin transformed by a smile and heavenly halo. But then the flowers fade and it returns to its fruitlessness, mud color and congested twigginess."

--TREES OF SEATTLE - SECOND EDITION