Lots of very good thoughts went into this -- thanks for all the hard work.
I would suggest reconsidering the notion that avatars not be visible by default. If indeed avatars are a good idea (and it would be great if we could somehow test that hypothesis), offensive avatars are likely going to be dealt with in much the same way that offensive usernames are currently dealt with, i.e. fairly quickly. If that is not the case, then I'm not sure an opt-out/opt-in mechanism is going to help much -- it just seems to add complication to the overall system that should be avoidable.