Aluminum, to me, is like TruStone in a way. What I have found is there is a lot more chatter when turning. If someone knows a cure I'm all ears. Other than that its real messy to finish. Wet sand with either H2O or 10W40. Either way you get a gray messy liquid that splatters everywhere. I got the satin finish by accident trying to get a shiney finish using 400 grit steel wool then top off with Ren wax. This worked for me I'm sure there are a lot more ideas out there.
The chatter is an easy problem to solve, Carbide insert tools, with a small radius, I love the shape, you nailed it, and yes, unless you used a hard anodizing it would scratch. Hard anodizing, isn't as attractive as the clearer colors, for a reference look at the brown aluminum and glass doors, your finish should age gracefully, one of the first pens I made was aluminum with a finish similar to yours, it's been used daily for over two years and a few refills, and it still looks nice. Great job!!!