Ozzie having a longer life span seems to be the only real possibility.
As for why no one in Gaurdia knows of Lavos there are a few possibilties.
We don't know the circumstances of the summoning. It is know, though, that he was not summoned in the same way that he was in 12,000 BC. This is obvious for he would have emerged from the ground and laid seige to the world. So if he wasn't summoned like that how could he have been summoned? The fight obviously didn't take place above ground in 600 A.D.
It is possible that instead of summoning Lavos, Magus summoned himself to Lavos, and the fight took place in the pocket dimension. I beleive this to be a very likely possibility.
Also, it is possible that Magus warped to 1999 and faught Lavos then. This may be possible, since it is rumored that Magus figured out how to travel through time on his own. If this were true though, why would Magus need the gate that was created after the battle with Chrono to return to 12,000BC? So, if Magus does know how to make his own gates it is most likely that he learned how after that event. For that reason I don't beleive this to be a very likely theory, I decided to include it though. Although the next theory could make the battle in 1999 true.
Another possible theory is that Magus could have only disturbed Lavos, and instead of Lavos surfacing, he may have made a gate similar to the one that was made by him after the battle between Magus and Chrono. Where this gate may have taken Magus, is really un-answerable. It could have taken Magus to Lavos in either the pocket dimension or in 1999. Or it could have taken him to just about any point in time.
The final theory I could think of, is that Magus was not successful in the original summoning. This is somewhat unlikely since Magus was supposed to disapear, but there are other ways that he could have disapeared. For example he may have just left the mystics. Why would he do this? The answer is quite simple, the may use for the mystics was to give Magus the time to summon Lavos. After realiseing he wouldn't be able to do so, he just left. There would be no reason for him to stick around if he couldn't summon Lavos, since the mystics obviously lost the war. Thus making this a somewhat likely theory, besides the fact that someone would probably see Magus eventually or he might not of given up on trying to summon Lavos.
Those are some theories I could come up with. You may have noticed that I said it is unlikely that theory 2 happened without the aid of theory 3, but I said theory 1 is likely even without the aid of theory 3. The reason I believe this is because traveling through time is obviously different then traveling to Lavos in his pocket dimension, since the pocket dimension is in the same time. Although the pocket dimension is still only a theory, but a very likely one at that. If there really is no pocket dimension it would still be different then warping through time.