The "artefact" (an alien ship of ancient and indeterminate design, possibly Voehn) was almost certainly nothing to do with the Dwellers or their mythical "Second Ship" that traveled from our galaxy to another and then back again.
The incident that spurred Taince to kill herself (and take Saluus with her) was that during their trip into this ancient ship, Saluus killed Ilen. Taince suspected that Saluus tried to get fresh with Ilen and Ilen clawed at his neck. In the ensuing fight, something happened and Ilen fell over the edge. The 'twisted piece of metal' has no special significance beyond that Saluus presumably grabbed it to save himself from falling or possibly to hit Ilen with.
'Let me explain, in case you still haven't got it: I saw the marks,
Sal. I saw the three red lines on your neck, before you put your
jacket collar up. Remember that? Remember pretending to shiver and
saying, "C-collar," or whatever it was? Remember? Just one of those
little false notes that you don't notice at the time because of all
the fear and adrenalin, that doesn't start to nag at you until long
afterwards. Kept that collar up afterwards, too, didn't you? Kept the
jacket on like some sort of comfort blanket until you could get to a
bathroom and a first-aid kit, didn't you? I remember. And when I was
reaching down to Ilen, I saw her fingernails. With the blood under
them. Saw them very distinctly. Fass didn't; still has no idea, even
yet. But I saw them. I wasn't entirely sure about the marks on your
neck, but then I checked. Remember that last farewell fuck, a couple
of weeks later? Just checking. They were very faint by then, of
course, but they were there all right.
'You always wanted her, didn't you, Sal? Always so desired the
beautiful Ilen. Did you think because she went into the ship with you
she was saying yes? Did you? Did she, then changed her mind? Doesn't
really matter, I suppose. I saw what I saw.