Well it looks like I'm finished. I did some final sanding at 320 grit, checked everything again, and put three coats of Satin Polyurethane finish on the chair (sanding lightly between coats). It came out very nice.
And for the most important test...
I weigh around 180 lbs (81 kg), and the chair barely flexes! It's very sturdy.
I hope you've enjoyed following along with the construction. Please drop me a note if you have any questions or comments.