ARM32 is not supported out of the box. See the following to build stuff for it:
Github Pages deployment is done by pushing to the repo and branch you specified (see https://github.com/travis-ci/dpl/blob/v1/lib/dpl/provider/pages.rb for the logic). So what multiple pushes will result in will depend on how Github handles such a case.
AFAICS Github Pages hosts everything on the branch tip as-is. So multiple parallel deployments should be okay as long as there aren’t any conflicting changes.