I see the main problem over here:
You are disheartened and panicking. Do a water change on yourself
. A bristle star being sucked up the powerhead and hermits molting should NOT let you down. There are cases where anemones get sucked up in powerhead and the whole tank gets nuked and still people didn't loose hope. Its all about taking up challenges. If something goes wrong, you should become more persistent to solve it than just giving up.
My POV for the high nitrate: the nitrate must have risen initially, the hermits started dying one by one and hence the nitrates just keeps on adding up.
There are loads and loads of great people out here who are willing to help, for example Denny (reefwars).