1) How does one remove/change the brushes?
2) How can I tell when it's time to do so?
3) How can I determine and/or set the advance?
Let's start here. http://hitorqueelectric.com/gallery/v/Step_by_step/Brush+Advancement/BRUSH+ADVANCE+PIC.GIF.html
1) As you can see in the picture, there are access windows in the CEH (Comm End Head). Brushes can be inspected and removed without disassembly of the motor. Although most installations have a coverband around the CEH to protect against debris which can be easily removed.
Each brush has a spring which can be moved to allow the brush to be pulled up. Each brush will also have a pigtail (wire) screwed to the cross connecter attached internally to the A terminal. Usually you can pull the brush without unscrewing the pigtail for inspection. However, you must unscrew it for replacement.
2) Inspect the brushes. They are usually good for about half the original length. Check with your motor supplier for specifics. In the typical daily driver EV, I would expect the brushes to outlive the car. Performance EVs where a 1000 amps or more are common occurrence, brush heat can cause accelerated dusting (wear) and replacement needed more often. Other factors can cause rapid brush wear such as contamination or dust (grit). So keep your ventilation air clean as possible.
3) See the link.
Another thing is that if a motor is in need of brush replacement, for whatever reason, it is likely to need the commutator reconditioned (turned and undercut). Like I said above, after 30 or 40,000 miles on your EV, maybe you should have your motor professionally serviced