Whether the appeal became moot after the insurer's reconsideration decision of 3 January 2017
Yes. The reconsideration decision annulled and replaced the challenged opposition decision, so the appeal lost its object.
Under Art. 53(3) LPGA, the insurer may reconsider a challenged decision until its response is filed. Once it did so here, the contested decision no longer existed, and the appellant could challenge the new decision separately.