No I dont think that just because you were once expelled from the land (We also dont truly know why they were expelled. I mean it could be because they were getting dangerous and already tried to take over the land but lost) you have the right to conquer it back, especially if lots of time passed between that. They might have had a bad start and had a bad place to live but this does not give them the right to conquer the world. Because in the end they did not just go back to conquer "their" lost land but also conquered lots of other lands.
In my opinion just because they might have started as a small nation with bad other factors around them does not change the fact that most of the portrayal ingame is evil. If a person goes around and murders people, they might get some sympathy because they might have had a bad childhood but this does not change the fact that they choose to murder someone. Even in their beginning they already partly used force to have other tribes by their side. And even if they tried to do it with negotiations in the past, the Galearns now are past any negotiations for trade and other things..all they want is the whole world under them and their belief. Conquering the world is not something positive and even if there are some good Galearns (I dont think that the common people are evil) the nation itself and what it stands for is not good.
I also dont really sympathize with the Ala Mhigans. At least not those that we mostly met until now. But I still like them more than the Galearns because at least they did not try to destroy a whole continent with a moon..
About the flag: But could a tight unity not also mean that anyone that does not fit into the unity might be disposed? Is a unity always just something positive? And having a flag that might symbolize some great traits does not make the nation behind the flag great. I am quite sure that we have lots of countries in the real world with nice ideals behind their creation of the flag but those might not always be the greatest countries.
I dont think that we will ever get to be true allies. Maybe there will be a time were they will choose our side (the enemy of my enemy is my friend kind of thing) but I dont believe that we will be as allies like we are to Limsa and the others. They would need to change the leaders of the Galearn and have most of the Galearns as sympathetic people. I believe that we might need to join forces against an greater evil but as soon as this is over we would probably just be enemies again. I mean how many true enemies do we have in the game? There is no real big other force like the Galearns in the world since most of the world belongs to them, so if they became allies we would not have a big bad human enemy anymore. We still have the Ascians but its imo already annoying that they are kinda behind any big bad thing in the game, no need to have them be it for every little thing..IMO they should just finally show that the Galearns really mean bad for Eorzea because right now we are mostly told how bad they are but dont really see why we should be afraid of them. I hope that 4.0 changes this a little.