Actually the Time Devourer is incredibly important to the plot because it's his fusion with Schala that makes Balthazar enact his plan to cause the Time Crash and send Chronopolis to 12,000 B.C.E so it would cause the Entity to pull Dinopolis from a parallel time and cause the events that would lead to the creation of the archipelago, Serge's birth, him becoming the arbiter, and FATE dragging him into the real timeline, and all the other events needed to free Schala. The entire plot of Chrono Cross is basically one giant Batman Gambit, who was only able to pull it off because he built a freaking city that could detect all parallel time lines. To say this game is convulated is an understatement. The entire point of the Time Crash and Serge becoming the arbiter was just so they could create the Chrono Cross and finally free Schala.

Yes, Lavos is the Time Devourer, its just a future evolution it takes after merging with Schala and being thrown into the Darkness Beyond Time., in between the events of Chrono Trigger and Chrono Cross. Sadly, the original Lavos is much more menacing, both visually and gameplay wise.