still have no idea

[Galuf]

how the Domans die. hell most of them are inside and i doubt they all drunk at one time. oh ofcour Cyan and Guy survive somehow

[FFNut]

It is implicated in the story later from the Emporor that Kefka poisoned them all and you can see Kefka in a cell. However I'd skip seeing him there as its a timed thing and he takes up 30 seconds of your time.

[Galuf]

we know they were poisoned but i dunno how he killed all but 2. which means it wasnt airborn

[FFNut]

It was put in the water supply so they I guess didn't drink the water. However to have that devastation he would need more in the river then a flask of it. Plus you would think the water would look off due to the dead marine life floating past in probably large clusters if it made its way all the way that way. More plot convince than anything I think.

[Sephex]

I think there might be a minor time skip or that poison spread in the water incredibly fast. Doma was on edge defending their territory for quite some time until Sabin (and Shadow, probably) showed up. Therefore, many soldiers were fatigued and trying to hydrate themselves to stay in good shape. It was just dumb luck and horrible timing that did them in. Again, I think there was a minor time skip for convenience sake to move the story along. It was implied that the poison had plenty of time to work through Doma's water supply.

[Kibbs Kibbard]

It took a while for the poison to filter through the Castle, also during war the soldiers probably drank from the river when they had a chance. The game makes it seem it happens instantaneously, but it actually took a while for it to take full effect. As for Cyan not getting poisoned, he's just a hard ass of a PC is my guess.