Here's my thoughts on the Fall of Guardia.
If Guardia fell, then the royal line would be broken. However, remember that Doan is a decendent of the Guardia line. So obviously someone survived, and it's clear that neither Crono nor Marle survived, as we see their ghosts in Chrono Cross. -CSM
Doan exists in the pre-Time Crash timeline. We have absolutely no idea if the Guardia line went on to Doan in the timeline featuring El Nido and Belthasar's influence.
I agree completely. To say nothing of the possibility of some member of the royal line escaping while Guardia falls without being Crono or Marle. Maybe a nursemaid took their newborn infant and raised him herself in secrecy.
....
So he can one day take back the kingdom, reconcile paganism and Christianity, and pull the Masamune out of an island due to the help of a time traveling Magus, who makes a round table for the King and his knights (One of these chairs being enchanted to kill whoever sits in it for some reason, and Magus sits in it and goes kablooie. The curse is broken by Serge, a knight of a pure heart, and helps the heroes find the Holy Grail, which is offered by Schala in her transcendant godliness) and then when the new king dies and leaves descendants, his evil half-sister fairy queen witch bitch will suddenly turn nice (WTF?) and carry his corpse to El Nido to be buried.
Oh, and my theory is that the CT ghosts are ghosts, but from an alternate timeline. Either that or the whole echoes of the past thing is a METAPHOR for ghosts, which are mere echoes of the once living person.