4) that would apply if you weren't trying to deconstruct the story and prove me somehow narcissistic.
Objectively incorrect, it applies no matter what the author's thoughts or the reader's intentions. Literally the last sentence in your post is made invalid through it.
2)The character himself is depressed at the start of the story, medically depressed at least. You claimed so yourself. Was he really in the original, first draft of the story? Because stories change with time, and depression isn't exactly the best feeling to replicate without experiencing it.
3)The "getting pissed at insults directed at the character" was not the main point of my previous statement. What I meant was that involuntarily you put yourself in the place of your main character and made him act like yourself, were you in his place. The character became alternate setting you some part through figuring things out. This is partly normal, characters will always have some of the author in them. However the RP and statements made by you all point to Satoshi having your morals, beliefs, train of thought and even the knack of making himself sound unpleasant by accident. Combine this with his job set(most of them being jobs you consider "heroic," and you greatly admire these "heroes"), his abilities(Immortal, incredibly adept at healing and combat), his ties to your favorite characters, the respect he garners... things all point heavily towards self-idealization, even if it is involuntary. And yes, involuntarily or unthinkingly putting a character one does not believe he self-inserts into into an idealized, near-omnipotent state is something that happens.
Shirou-wanking is a prime time example for this. And despite news to the contrary, a lot of people self insert into Shirou. What helps is the first person aspect of it all, constantly being in his head, and the watering down of his character aspects in Fate, the route belonging to the most popular heroine and by extension the route most replayed. And people don't believe they self insert into him because "hey he's fucked up," they say consciously to themselves to deny themselves. And yet, Id, Ego, Superego.
You are doing the same multiple times over, idealizing what you've inserted into with great power, but as an OC, and with your mindset concerning the nasuverse (Hey, on thread topic!), you believe you have zero limits and made him unkillable and amazing and all stuff I've mentioned before.
And also, defending your character against people telling you he's bad is MORE likely to make you insert into him than before, because his is the role as someone being kicked down (by gilgamesh) and fighting back, just as you are fighting back against "haters".