As to the wrong forum, this is not a mod..
Well, it's not a community game or story either. You'll get more help in the modding board, because that's where the people who know how to manipulate the game's systems hang out.
So, unless I'm mistaken, your problem is that humans (and other mortals, like dwarves) continue to become "obsessed with their own mortality and seek to extend their life by any means" despite their being no actual ways to immortality. I'll just offer my thoughts:
1) I don't think this is as big a problem as you're making it out to be. AFAIK being obsessed with mortality doesn't interfere with them doing other things or pursuing other goals (such as "dabbling in local politics" or "getting married and having children"). As it stands, it's more or less just a flavorful way of getting people to learn secrets. So if it bothers you that much I think you can just ignore it, because it doesn't really affect the world in a meaningful way.
2) To expand on Cathar's comment: in real life humans have often - pretty much always - sought for ways to extend their lives through magic: alchemy, searching for the fountain of youth, etc. etc. And they did these things despite the fact that, as you say, "it is not possible for humans to live forever in the real world." But they didn't know that. Similarly, your DF world may not have any actual secrets of life and death - but your humans don't know that. So, to be honest, your situation actually sounds extremely realistic.
3):
these humans do not seem to understand this universe I'm trying to build.
Welcome to DF modding/worldgenning. The game will fight you at every turn. The more specific you want a world to turn out, the more hardship and suffering you'll have to endure to make it do so. And some things just aren't possible -- this may be one of them. A lot of stuff is simply hard-coded and can't be altered, and mortal races seeking secrets of life and death might be one of those things. But, as I said above, I don't think it really interferes with much, so my advice is just to ignore it. However, do ask the modding board, since they're likely to know more.
Also, are you running any mods on this world? I ask because "copper golems" are not a vanilla creature. If you are, it's best to tell people that up front, as sometimes it's the mod causing trouble, not DF itself.