Patur Aval Assur: I did not read the entire thread, but I would like to answer your original question. I believe this may be correct answer, so please read carefully.
Your question is based on the assumption that all humans only do things that are motivated by a pleasure that they will derive from that action. Your question follows that “why” should we serve Hashem for any reason other than reward (which the Mishna expects us to do). You cleverly pointed out that reasons like feeling good are really only other forms of reward which should not be considered different than doing it for the Olam Haba type of reward.
When you ask “why” should we do it, you want an answer that will contain some sort of pleasure based motivation that will explain the action, and then you refute it by saying that it’s reward.
But your assumption is wrong. The question why should or why would a person do something does not necessarily make sense. Think about it. How do you know that people only do things for a reason? Does a plant decide that it’s a wise idea to start growing in the spring? How do you know that people are different? Observation? I am not arguing with you that it certainly seems that people behave that way, but how do you know it’s an inherent rule of nature? There is no known scientific law that explains why people (or animals) must do things only for a personal gain.
Now, I am not suggesting that there are people out there who bang pots on their knees every morning for no reason at all. But as a Terutz to explain the Mishna, I would like to suggest the following. There is a Madrega that can be reached at which point a person is serving the Ribono Shel Olam not for the purpose of getting a reward. Which includes all reward. “Why” is he doing it? Because he reached that level of Avodas Hashem where his body follows the Torah even if his bodily desires contradict it. His body no longer follows the rules of regular bodies that only do things for the purpose of pleasure.
I don’t think there is a logical argument that can be made against the possibility of such a thing existing. It might still bother you that “how” can he be doing something with no reward type of motivation. I will answer that perhaps if we continue to serve Hashem non stop in the most extreme way possible for us, maybe one day we will experience the true Avodah of Shelo Al M’nas Lekabel Pras and we will see for ourselves exactly how it is possible.