I’ve been following this since the beginning, my first thought is that being up against the city, you’re not going to get anywhere without getting the media involved (while we’re at it, make sure you document everything, every phone call, every email, every person you talk to along the way). And, FTR, I’d probably, conveniently, ignore the plat the golf course has. It’s not official, it’s not sealed, it’s not filed with the city and it’s, more or less, just dumb luck that you found it. If asked, you can say you recieved it long after you bought the house (btw, mark the date you had it in your hands). Besides, how should you know this is the correct one, shouldn’t the “official plat” supercede an unofficial plat a private golf course has on file.
My second thought would be to just build over it and if the city wants to get under it, say 'what pipe? there’s no pipe down there? I have the official plat right here. It doesn’t say anything about a pipe? (remember a manhole could be for a pipe running in any direction and it was covered when you moved in, the pictures of the house from when you bought it, if you’re lucky, might even show that area. I kept the the pictures the the previous homeowner used to sell his house (by owner) because they showed some illegal work the he did). If something ever becomes of it, I’ll have proof that it was there before I moved in. Assuming he doesn’t lie about making the flyer.
Another thought would be for you and the neighbor to get together and have both your insurance company’s deal with the city at the same time. Between the two of you and the two of them, the extra pressure might help them move it. It also might help if you could do some due diligence on your own and maybe suggest some alternative routes for it (hey guys, why don’t you just cut right to the street).
Honestly, I think you’re going to end up having to sue the previous homeowners, they received money from the city for the mistake and they may have signed someone stating that they were to disclose this. On top of that, I can’t see a scenario where the city is going to pay out twice on the same mistake. My WAG is that when you go after the city, this is the first document that’ll be produced. If you’re lucky, you can sue them [the previous homeowners] for more than they got since you wouldn’t have even bought the house if you knew about that sewer line.
Like you said, start with your title insurance and go from there. If you had a lawyer present and the closing, that would be the next logical step. He/she can verify that this was never disclosed and isn’t buried in the paperwork somewhere.