As for a resistance I think that the Returners win on the sole fact that they stuck by what they felt was wrong, meaning they didn't really add to the already growing problems of the world. They were able to remain, for the most part, under the eagles eye and also able to have people in various places be apart of their fight without being noticed.

Where as AVALANCHE had no problem adding to the growing problems of the world because it was essentially for the greater good. Bombing reactors, taking down plates, things that would essentially change others lives for the worse where as they (AVALANCHE) may see it as a necessary step to make further progress.

Forest Owls... xD

Oh and as for the resistance. Like out of the cast of FFVI, everyone but strago, relm, shadow (and the stupid side charas) pledge their help and what not to the returners, thus keeping the returners alive until the end of the game. If you look at the actual characters AVALANCHE had, yeah they did die, but with the returners Banon was really the only named character that was in the returners. Once you get to the point in the game where they all pledge their help, they can be considered the returners as well.