I don't know, Beck, it seems like you're answering your own questions (and you asked a few). A stiff upper body is bad news in all the scenarios you mentioned.
The first step towards relaxing that upper body is to realize that you can't really force your bike to stay upright by being stiff. If anything, the bike will stay upright
in spite of your upper body being stiff, not because of it.
Wiggle your fingers, waggle your elbows, be aware of stiffening and consciously direct yourself to relax, and to support your upper body with your core muscles rather than your arms.
When riding over uneven surfaces, such as the pavement on Redwood, keep your eyes up and looking out in front of you. Fixating on those grooves, staring at them, and stiffening up is like a self-fulfilling prophecy. By doing those things, you are creating drama where there is none. Keep looking ahead.
When pulling off pavement and on to gravel, such as to stop on the roadside, do most of your slowing on the pavement. Once on the gravel, depend more on your back brake than your front, but whichever brake you use, apply it smoothly, keeping your eyes up. You'll be fine.
Rich Oliver is having a women's camp this weekend, I think. Too bad you're not there. He'd make all this stuff easy-peasy for you.

On that note, sign up for an MSF dirt bike school. One day only. They provide the bikes and gear. You'll learn a little about riding in slippery conditions that will help deal with these fears. Maybe you'll even find you like dirt bikes. Riding in dirt is a great way to become a much better rider.
