The salinity at 1.028 is on the verge of being a problem for the fish if long term. If you have no coral then it is fine to keep it around 1.022-1.023. Along with the benefit of less aggressive parasite issues, the lower salinity makes it easier for the fish to do the whole osmotic balancing act. The higher the salinity the harder their kidneys have to work, and long term it can be problematic.
Having said all that, unless the salinity was that high for a very extended period of time, I doubt it was the cause, and for sure it wasn't because you lowered it too fast otherwise the effects would have been noticed within minutes, not a week later.
I also have doubts about the nitrate level being the culprit. 80 ppm
is not drastically high at all for fish, inverts and coral yes, fish, negligible impact.
One thing to consider is the trigger. Were they roughly the same size or trigger smaller a years ago?
Is the trigger now the same size or bigger than the lion was?
Undulated triggers are one of the meanest and territorial of the family and I would not be at all surprised if as any size differences became less, that the trigger began exerting it's dominance and may have been stressing the lion and keeping it "under it's thumb" so to speak.
I've seen it happen before, where a dominant fish won't kill the others, but it won't allow them to eat or roam the tank freely until they die from stress.
The few undulated triggers I have had experience with all ended up as the only critter in the tank.
and as a side note, feeding fresh water feeders; guppies, goldfish, to salt water fish presents very, very little possibility of introducing parasites. But do not use any live saltwater fish as feeders because they will be almost certain to have something undesirable on/in them.