I wouldn't worry about wildlife - they are more afraid of you than you are of them and getting to see a wild animal is a treat. I would worry about terrain. The understory in the foothills of the Cascades is really dense and hides rocks, small cliffs, and all kinds of obstacles and hazards. When you're by yourself you don't have many options if you get hurt. I would also worry about other people. If you stick to 3-5 miles out of town you might as well be exploring in a greenbelt. There are other folks who find the woods to be curative and some of them cook meth or do other things they don't want anybody to see - and even without considering illegal stuff you're asking for trouble if you stumble on somebody's camp.

I've done my share of off-trail travel out here and I understand the attraction. You'll find it much more socially acceptable if you present it as goal oriented - pick yourself a list of peaks to work on and start knocking off the easiest ones. It doesn't matter if you actually summit, but for what it's worth most of the minor peaks in the Cascades are non-technical and the approach is 90% of the challenge. You'll still get some pushback about solo backcountry travel but you'll find it a lot easier to explain and a lot more acceptable.

Vashon

if it's not a "trade or business" it's not going to be taxable income. If it is a business it would not be a capital gain - the resale price would be ordinary income, against which you would take your expenses including the original cost of the car.