Well, IMO, Ichigo only cares about his friends and his town. Although he knows that Urahara used him and Rukia in some sense to fight against Aizen, he also knows that Urahara helped him to become stronger, strong enough to defeat Aizen (for the purpose of protecting his town and the people in it from being destroyed). Moreover, he was helping Karin out unknown to Ichigo when he lost his shinigami powers.. What Ginjo was telling Ichigo to do was to doubt his family, and I dont think Ichigo will ever do that.
It doesnt make sense that a king's servant is located in a different world for more than a century. How does he serve his master like that?
Although 'Soul King' sounds like the almighty being, his existence might not be of harm, i.e. villain. From Aizen's conversation with Urahara, I concluded that the soul king might be a form of existence that keeps things in balance. We do have such 'things' IRL, essential existences in systems around us. Because these existences keep things in balance, they sometimes are perceived as 'harmful' or 'unfair'. Because we do not know what excatly is the soul king (since Urahara refer to the soul king as 'it', we cant say that the soul king is a person), we do not know why Aizen resented 'it' so much. An analogy that I can think of (not sure right or wrong) is capital punishment. It is the most extreme form of punishment in modern society. Many are hugely against it, saying that it's inhumane etc etc. However, there are those who accept it as a balance in society, its existence is essential to punish the most cruel and unforgivable criminals. IMO, the soul king is smiliar to such an existence.
So what I am trying to say here is that, the soul king might not necessary be a villain, although 'it' might turn out to be depending on perspective. That is because Aizen did all his evil deeds in order to destroy/kill the soul king, it shows that in some way, the soul king is 'bad'. But Urahara did say that the existence of the soul king is essential for SS. So, in the end, if Ichigo were to really fight the soul king, I wonder how SS would become...?