Browser back button works fine in chrome on a Android with amp pages fwiw. Does safari mobile not support the session history management APIs? caniuse says it does, but it sounds like it doesn't work from what you're saying.
edit: I can see it. Next iteration you will need a AMP key to publish your content and clients will need AMP key to consume content! Browsers that don't include the key will be excluded from the ampweb.
Ironic, as toyotas have more made in USA and assembled in USA cars and car parts than ford does, and has for over a decade. Only Honda or Chevy can claim more american than Toyota iirc.
