Just a quick not on the "How many people can Crono fight at once and survive" bit.
Saying that Crono can fight 20 people himself may be a bit misleading.
First, you have to consider their power relative to Crono's. We can assume that they aren't level 1 townsfolk. They are likely magic weilding, as use of the elements is typically considered the catalyst to Porre's victory.
Then you have to consider, are they all lining up to be fought one at a time? Or are they attacking at once, possibly using short range, long range, or a combination of both kinds of attacks.
While the battle system demonstrated 3 people taking on the power of Lavos and surviving, that is not the equivilent of taking on 20 people. I think 20 decently leveled people in one battle, would be a bit of a challange for Crono. Imagine a normal Chrono Trigger battle, only with 20 decently leveled magic casting enemies instead of two or three, and with one party member instead of three and think about how much of a pain that would be to play through.
The response I expect to get is that the battle mechanics are for gameplay and therefore cannot be seen as a true representation of how a battle would really turn out. That is exactly the point I'm trying to make. We really don't see anything aside from story that we can use to determine his sucess or failure under such curcomstances. And story seems to point heavily in the direction of the latter.
EDIT: Just wanted to point out, I'm not saying that he died. Just that I don't think the idea of Guardia falling despite its high powered defenders is as hard to swallow as it might seem at first.