Sorry to bump this but, have all of you forgotten that fallen Guardian troop with the note about the Jugglers inside the Magic Cave? This shows that Guardia (or some other army) attempted to infiltrate Magus's Lair and ended up retreating.
About Zenan Bridge, we never see the Knight Captain fight, perhaps his power rivals that of Crono's party? Of course in the Original or Lavos timeline Frog saves Queen Leene and helps defend Zenan Bridge. Perhaps the battle turns out much differently then when Crono interfered. Maybe Ozzie as well as Zombor was defeated in this battle. The Mystics losing Ozzie, as well as later losing Magus to Lavos would very well explain how Guardia won.
Also on another note, the reason why I think the Masamune broke on Cyrus was because it hasn't been tempered with dreamstone for possibly 12,600 years.
Also, Glenn and Cyrus have battled Slash before, the circumstances of this battle we do not know. It possibly could've been in Choras, or even in that "Magus's Lair invasion" I speak of.
My thoughts also on the Frog King and Hero Medal, I believe that the Hero Medal was kept in Guardia Castle before The Frog King got hold of it somehow, possibly through taking it under Magus' orders.
And my final theory for this post, about the Acacia Dragoons, perhaps the 1st weilder of the Einlanzer was, Frog. This sounds a little unlikely but I think after he returns to his human form he takes Tata and trains him. I used to think that Frog returned the Masamune to the Denadoro Mountains when he was ready to retire and everything. But now I think he passed it on to his kids, if he had any or even Tata when he was older.