I have two theories as to elf immortality. In both cases it didn't apply to all elves but just to an elite.
The first theory has elf mages co-existing with spirits and the first of these were in fact the Creators that the Dalish now think of as gods. This is why their legends talk of the Creators teaching them how to do things. Also why the Dalish think all elves were magical, because their knowledge is limited in what they know and they also believe that elves with magic are closer to what their ancestors were (which is why Keepers who are always mages are held in such high regard). When humans first appeared the elves largely dominated them, possibly even used them as slaves, and this led to tensions between the races. The co-habiting spirits were repelled by the bad vibes/negative emotions and retreated back to the Fade. As a result the elven elite started to die. This caused more widespread tension, so more spirits retreated and so on. Being linked to a spirit does seem to be able to prolong life; even Zathrian's life span was tied to a spirit.
The second theory has to do with the elven dreamers and Uthenara. Again it is the incomplete knowledge of the past that led the Dalish to believe their ancestors were immortal. However, there are aware of Uthenara so it seems unlikely that this is the true cause for the belief in immortality. So my money is on the spirit link.
As for the Golden City, it is possible that it was a reflection in the Fade of Arlathan. When the Tevinter Magisters sank Arlathan underground it is possible it became corrupted (by red lyirum?) and this was reflected in the Fade city. Whoever encouraged the later Magisters to try and enter the Golden City had maintained the illusion it was still golden when in fact it had already changed; when they touched it in the Fade, they encountered the reality, broke the illusion and it transported them into the actual corrupted city underground where they were transformed into darkspawn. My main reason for thinking this, is the eluvian in Origins where Tamlen said he could see a black city and out of which came darkspawn. There was also hinted to be some sort of evil entity within the city and this might well be an elven dreamer who was trapped there when it sank and by pretending to be Dumat, was able to lure the Magisters to their doom and also ultimately reek revenge of the dragon gods who aided in the destruction of Arlathan.