Your theories all seem quite plausible. However, let me set up another one.
Those ships roam space for decades. In the case of the Thetis, I understand it was even more than 100 years. A ship crowded with People separated from the deadly void by a thin wall. This surely wasn't the first occurence some of them got mental, even if it wasn't to that scale.
In a tense athmosphere, there is not much needed for a "natural" mass psychosis if all conditions are met. Let's assume the ship had already survived several internal struggles which are unavoidable. Factions rise inside, much as you would expect it in a prison. Nothing distracts otherwise from this dull, never-ending journey.
Then suddenly, sensors record an obviously artificial (crafted) Sound coming from a planet. Fear rises from the unknown. Tinfoil theories sprout from everywhere, rumors as well. Then a (maybe unrelated) Event happens, like one can unfortunately happen in every Society: a murder. Maybe witnesses have even seen the murderer behave like crazy. Not much differently from those who went mental before, but there was this Event before, you know, that strange recording, everyone heard it through the voice-com.
Maybe the murderer had to be shot down by the security guards, so he didn't have a Chance the explain his deeds.
So the tinfoil-hats gather to admit that it was the recording that got this poor man crazy. And as everyone else heard it as well, it's a matter of time before someone else gets killed.
The clear-minded mission-leaders try to curtail the rumours in vain. People get even more suspicious of each-other as it was already almost traditionally the case.
The self-fulfilling prophecy is at reach. It doesn't take Long before someone kills someone else, aledgedly in self defence because that guy had a butter knife in his Hands and had a weird look on his face. This Event is enough confirmation for that tinfoil theory to most People, and such Events happen again and again, at an increasing rate as more People arms themselves and everyone seems to think it's better to kill preventively than to wait and see what's that guy's intent with this fork.
Eventually, only few are left. In the meantime, the theory is confirmed in the mind of everyone. No one doubts it anymore. No one thinks of a mass psychosis. Therefore People submit to their imaginary killing Impulse much faster. When the last People remain, they park the ship in the nearest available Orbit and leave a message to those who may find the ship one day. A message that in reality contains a false assumption.
The last inhabitant eventually dies of decrepitude or solitude, or a legit and understandable crazyness that makes him commit suicide.
Hundreds of years later, we find the ship and its message and start to look for a Signal that maybe was just statics from a decayed human probe sent out centuries before even the Generation ship was lauched.
Since the record doesn't seem to have much effect nowadays, I really tend towards this theory, even if it is much less... exciting.
!
