You should finish it, along with the next one - they're worth it, trust me.
Yes, Matthew's mom was Jenna; she was one of the main characters of the previous games, though she was mostly playable on the second. It was weird when I learned that she married Isaac in the end, because throughout the games, it was hinted that she and Garet shared a crush... heh, guess that was just teenage stuff.
I don't know, really. In the definition of the world of Weyard, it states that it is made of the four elements. If there was another element, it has to be other-wordly, like those monsters in the Eclipse's shadow.