Nice post, the Mutianyu section is best to avoid the crowds and vendors (still a few of both) but nothing like the other sections. If you are really looking to get to the overgrown parts you can do some cool tours with the Beijing Hikers. The bus information is good, because the tours might only give you an hour or two at most and maybe ‘forced’ shopping (which is not fun). The wall is really amazing!

Thanks,
Miguel