Vixen
Penultimate Amazing
"If". With no evidence of smuggling on that voyage and obviously none of a pointless submarine escort, that's a very shaky "if".
What "if" the Russians had already threatened to sink it as you like to speculate? Takes the heat off an inside job, doesn't it?
Nothing about it explains "disappearing" the officers.
The Russians (or whoever it was) cannot do it without insider help or to knowledge that something was 'irregular'.
The Captain rang Stockholm ahead during the journey to say there was an extra consignment on board (=lorry). The Customs guys want to know this in advance, not in the middle of the journey.
If the Russians were 'tipped off' then that too, is insider knowledge.