The mobile support problem could be solved by a carefully planned transition
to Drupal 8:
https://www.drupal.org/drupal-8.0/features#mobile
Drupal 8 will presumably be released sometime next year (first beta was just
released), in which case most maintained modules should be compatible by the
end of 2017, which gives the Trisquel webmasters about two years to plan and
prepare for a smooth migration. I feel that would be a better use of their
time than setting up a whole new forum package, and learning how to support
it long term (if they don't already), but of course that's up to them.
BTW: Just for the record, Discourse does allow replies by email:
https://meta.discourse.org/t/new-reply-via-email-support/7764
