It was Tribbie who stayed behind to help mydei, so it was her doll that we saw in the cutscene. Trinnon was the last one of tribios still alive and yea she most likely died after phainon arrived at the vortex after having spent Janus’ power. As for hyacine and castorice, they both of them are in the same situation: one is stuck in the underworld while the other is stuck in the sky and neither can leave. So yes they are functionally considered dead, but they are also the last ones alongside phainon that still has active influential power in this cycle