[...] Or maybe there was a tip-off the Swedish SäPo were waiting for it in Stockholm. Note how all the senior officers mysteriously 'disappeared' after being rescued.
[...]
Add to the fact Sweden wanted to bury the whole thing...
I really like this theory. It has potential.
Sweden caught wind of illegal smuggling, and they were ready to bust the smugglers as soon as they got to Stockholm. The smugglers (who may or may not have been the senior officers) somehow found out and sank the ship at Swedish midnight (maybe intentionally, maybe accidentally), killing 852 people. After that, Sweden decided the smugglers were alright. They gave up trying to arrest them and started a decades long, globe-spanning conspiracy to cover up the truth.
This one is plausible. Any new investigation should check into this story.