Not to mention that Omoikane is basically the god of Knowledge/Wisdom. It makes sense that they ask and follow her advice, if you don't then you're probably against her in the first place.
As for the question about the Kami, I can't explain much but here's what I know.
There are considered to be three main variations of Kami, Amatsu-kami ("the heavenly deities"), Kunitsu-kami ("the gods of the earthly realm"), and the Ya-o-yorozu no kami (八百万の神, countless kami, "八百万" literally means eight million, though they meant it as "many". These classifications are not considered strictly divided, because of the ever-changing nature of Kami, but they are held as guidelines for grouping Kami.
In general, Amatsukami refers to kami residing in the Plain of High Heaven (Takamagahara), together with those that were born in Takamagahara but later descended to the land of Japan. The Kojiki discriminates five kami as existing in the heavens, namely Amenominakanushi no kami, Takamimusuhi no kami, Kamimusuhi no kami, Umashiashikabihikoji no kami, and Amenotokotachi no kami. These five it calls "separate kami of heaven" (kotoamatsukami). The term "separate heavenly kami" originates in Kojiki itself, which states, "The foregoing five kami are the separate heavenly kami." These five kami are characterized by the fact that they came into existence alone (hitorigami), and after coming into being, "hid" themselves. After them are just called Amatsukami. The main difference, as mentioned above, is that they were created or based from something.
Example of this group: Izanagi, Omoikane, Tsukuyomi, Amaterasu, Susano-o, etc.
The Kunitsukami are earthly kami in Shinto tradition, and they also serve as earth spirits that protected humanity and provided good fortune (although it is known that kami in general were more like nature spirits, the kunitsukami are the closest to that). They once ruled over Japan until the Amatsukami (heavenly gods) expanded their influence. Though most of them are Native Gods/Spirits, a human can become a Kunitsukami. Okuninushi was a human if I'm not mistaken.
Example of this group: Sarutahiko, Okuninushi, Sukuna-Hitona, etc.
As for the Yaoyorozu no Kami(hey, the first link is Touhou, because why not?) are pretty much any Kami that came to being in the world, since virtually any thing may be kami, if it makes us feel the existence of supernatural or extraordinary power and impresses us with a sense of awe, regardless of whether it is "good" or "evil".
Taken from third volume of Kojikiden:
In general, kami refers first to the manifold kami of heaven and earth we see in the ancient classics, and to the spirits (mitama) in shrines consecrated to the same. And it further refers to all other aweinspiring things?people of course, but also birds, beasts, grass and trees, even the ocean and mountains?which possess superlative power not normally found in this world. "Superlative" here means not only superlative in nobility, goodness, or virility, since things which are evil and weird as well, if they inspire unusual awe, are also called kami.
Example of this group: mountain gods, the god of rice fields, the god of toilet, god of kitchen and god of the rice grain...you know the drill. It's everything anyway!
From these, I'll put Kanako(since she came from Yamato, which are said to be descendant of gods and worked under the orders of Amaterasu) and Eirin into the Amatsukami, Suwako into the Kunitsukami, and that god of metal that Reimu summoned in WaHH into the Yaoyorozu.
Also, age doesn't really mean much to these kami. Take Amatsu-mikaboshi, he's even older than those Kotoamatsukami but is only considered a minor god, since he never do much anyway. So I think there's no reason for Kanako to follow Eirin's or even the Watatsukis' orders around. Kami are free to choose who to listen to and give their help. I think the reason Amaterasu helped her was just because she wants to.
Correct me if I'm wrong though.
