So, is it accurate that Dave could have repaired his swordkind specibus, (seeing as John was able to repair his hammerkind when it was broken into handlekind) and he just didn't?

Yes because of his time swords. If a weapon is altered while in use like when Dave’s sword broke the specious changes with it to avoid leaving the user unarmed. Dave created several half-swords that had temporal abilities allowing them to move along their own timeline to before they were broken or forward to when they were, effectively allowing Dave to change his bladekind into halfbladekind at his disgrestion.