Echo most of the above. It would be nice to have a damper at the vent but thats probably not your problem.
Vapour barrier should be installed around the fan as it should be on the warm side. Make sure it and all the ducting coming off is well capsulated by the insulation. One of the first pics you posted showed the insulation was a bit patchy but I assumed you mad moved it around while investigating. If the body of the fan is exposed to the attic air it will be cold and be a place for condensation.
While those openings aren't pretty they are typical. No staining on the sheeting so probably not a point of ingress.
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