In Maryland you are allowed to prune anything that is hanging over on YOUR property. Don't know about where you live but it surely would make sense. I would check with the authorities and see if you could legally do this and if so then DO IT. Prune your one tree to give more light. There are a lot of things that you could plant that take shade and moisture. check this catalogue for starts http://www.plantdelights.com/Wet-Shade-Plants-for-sale/

I also live in Washington and you are allowed to trim anything that hangs over your fence (as my neighbor explained to me as he butchered back my trees and damaged them.) Go ahead and do it - just be thoughtful of how you prune and don't do what my neighbor does and throw the trimmings back into my yard in a fit of anger because I block his light.

OK -- a dose of reality here:-)) I also live in western WA and unless you are in the middle of the Olympic rain forest, it does NOT rain 9 months a year here. In fact, rainfall through most of western WA is considerably less than in many other parts of the country. And it is also a recurring summer drought area with minimal rainfall accumulations from end of June into October. Even drier in eastern WA.

Second, conifer (like the 'cedar') needles do not make soil acidic. That is a gardening myth. Acidic soil conditions are determined by the underlying mineral content of the soil and amount of rainfall, not by plant material. The plants grow there because they like that sort of soil. Most of western WA has moderately acidic soil - somewhere around 6.0 to 6.5 or so. The good news is that the vast majority of plants will prefer that sort of soil condition. So check acidity off your list as a problem.

The primary issue with big conifers (or even our own native bigleaf maples) is that they produce shade and massively spreading root systems that hog soil moisture and nutrients. This is a dry shade situation, not wet shade. And there's scores of plants that will work in this situation, including many wonderful natives like sword fern, deer fern, western columbine, western bleeding heart, Oregon grape, salal, huckleberries. And there are many extremely attractive non-native plants that will work equally as well - Euonymus fortune, Sarcococca, shrub dogwood, bigroot geranium, Iris foetidissima, epimediums, hellebores, etc.

As I am a consulting horticulturist and garden designer in the middle of woodsy Kitsap county, I encounter this gardening dilemma on a daily basis. And you should see some of the gorgeous, lush and colorful gardens I see under just the circumstances described above. It is just a matter of understanding the conditions and then selecting the right plants.

Have you ever used a pole saw? I use that for trees limbs that are too low and hanging in my air space.