Trees and shrubs aren't my strong points, but I know that tulips won't last long in a north-facing yard! They need full sun (south or west-facing) There are scads of shade-loving perennials that you could plant around your trees. Hostas, lamium, lily of the valley, monkshood, periwinkle (Vinca minor), siberian irises, ostrich fern, bleeding heart, solomon's seal, snow-in-summer (less invasive in the shade), auricula primroses, bishop's mitre (epimedium), astilbe, jacob's ladder, martagon lilies, heuchera, more hostas. . . all of the above are hardy to zone 3a. (I'm in Edmonton and have a shade garden). Good annuals include lobelia, pansies, ivies of all kinds, coleus, caladiums, your houseplants (bring them back inside before it gets cold in the fall), impatiens, begonias. . . One tree I'd get if I had a yard would be a mountain ash. I'm pretty sure they can take shade, and they've got amazing year-round interest with their reddish bark, unique leaves, amazing fall colour, and berries that feed the birds all winter. They also don't drop their berries nearly as much as crabapples, so you don't get the mess. And I second the suggestion for junipers over cedars. A juniper will simply do WAY better for you. Calgary has some really great nurseries though, so you could also go visit some and ask for their suggestions. I know that the chinooks offer, shall we say, 'unique' points to consider. =)

Yes, that should be fine and enough to produce good bloomage. I grow some hydrangeas on the north side with no direct sun; the area around the plants is brightly lit by the sun though so, I would say my shaded area has a "bright" shade. The space on the picture may be tight for many hydrangeas though. But it hard to tell the dimensions from the picture. Measure how wide can the plant be without touching the existing shrubbery but assume the existing shrubbery has reached their mature sizes. For example, if those shrubs will get 1 feet wide when mature and that shrinks the available space down to 3 feet, look for hydrangeas that get no wider than 3' when mature. You usually want the hydrangea's width to fit in the planting space with some leeway... some separation between the hydrangea and the other plants. This improves the air flow between plants and minimizes fungal infections on the leaves.

'Obsession' Nandina? Looks to be one of best looking ones for Nandina. Maybe Encore Azaleas if you're willing to put in effort to build raised bed for them if you have a tree on the west and east side to provide some shade during the summer. You know it's basically all shade half of the year though...

Where in WA state are you located? Are you new to the area? It is not as wet as most think (although this winter has been unusual) and actual annual rainfall amounts are rather low compared to many other areas. It just occurs at a slow rate over an extended period of time so it may seem like it is wetter than it really is. Summers are typically very dry. Without any overhangs to block light, a north facing planting area is not all that dark. It usually receives quite bright but indirect light so one is not overly hampered by limited plant choices. You just need to avoid things that require full sun :-) And if living in western WA (if rainy, that would be my first guess), we have a very mild climate that does not limit plant choices, including a whole host of attractive plants that would appreciate something less than full sun. If you want a more hands-on design plan with an appropriate plant selection, consider hiring a design consultant for a few hours. Many local independent nurseries will have referrals for qualified individuals if they don't offer that type of service themselves.