To answer a question you asked. Due to how remote the water tribes are, being located at the north and south poles, and the fact that the airbenders were wiped out, the war has practically been the earth kingdom vs the fire nation for 100 years.

Fortunatelly Aang has a little more time then just a summer. As this was the winter solstace he basicaly has little over six months till the end of summer. Still crazy but better.

The biggest mystery is how iroh was able to see Roku's dragon

The spirits in this world are (like a lot in the show) inspired by east asian cultures and history, so they are more like nature spirits, like in chinese mythology. The spirit world is a world that coexists parallel to the human world.

Iroh can see the spirits because he made a previous trip to the Spirit World and became a spiritual person. Aang is the reincarnation of all the previous Avatar before him, Roku and more, so he can talk to his previous lives. He just don't know how at the moment. I can't tell how he can access the other elements without learning it at the moment because it will be explain in season 2. The normal way is by learning it in the right order. Even if our heroes are in a rush, Aang is the Avatar who is the master of the 4 elements, don't forget it. You would see it sooner.

Hopefully, I can help clarify some things. The Winter Solstice is the day with the least amount of sunlight and marks the first day of Winter. However, the fact that they are traveling from the South Pole to the North Pole does complicate things a bit: In real life, winter and summer are reversed on opposite hemispheres but I don't know if the Avatarverse does this or not. Temperatures near the equator tend to be hot year long but they instead have a dry/wet season. As you get close to the poles, temperatures are colder year long (e.g. 60 degrees could be the warmest summer temp). Obviously, the poles are always cold but there's a huge difference in sunlight between winter and summer (I don't think the Avatar world does this). The reason I mention a brief summary of climate related info is because I've noticed many reactors become impatient and feel like the Aang gang aren't making any progress. If you look at climate/environment and dress cues you can estimate how far along they are (i.e. Sokka/Katara wear a coat with fur, someone mentions a wet/dry season). Hei Bai isn't so much a ghost (deceased entity) but more of a nature spirit. In this case, Hei Bai is like the living embodiment of that forest (if the entire forest was destroyed, Hei Bai would probably be destroyed as well). It's possible that Hei Bai took it's anger out on the village because that village was supposed to take care of the forest in exchange for Hei Bai protecting their village and the forest providing for them. This is a common motif in animism and nature-based belief systems. I'm not sure if you got this bit of info from an earlier episode that Roku is Aang's previous life or Aang is the reincarnation of Roku (same thing). The Avatars all share the same spirit that reincarnates to the next Avatar after the current Avatar passes away. It can be difficult to get the metaphysics of it but you have to understand that all the Avatars are both the same person and also separate people at the same time. One of the challenges is accepting as a viewer that you kind of have to suss out which situations have a higher degree of sameness and which have a higher degree different-ness. For example, even though every previous Avatar has already learned all the elements before, Aang still has to learn them all again (this situation highlights more how the Avatars are separate people). As you have gleaned on your own, when Aang's eyes glow, there is a greater degree of connection with other Avatars (this situation highlights more how the Avatars are the same person). You sort of have to try and figure out what the 'rules' are in this world as you go along and I'm honestly not sure how consistent they are with the 'rules' but there are other factors to consider that influence the connection between Avatars (e.g. maybe the connection is higher in the 'Spirit World'; maybe there's a greater connection as Aang becomes more spiritually adept (I don't know the answer to these, they are just a couple examples of things I think are reasonably possible)). Unfortunately, with a complex concept like reincarnation, this type of complication is unavoidable.