Don't be so sure. The Strategic Bombing Survey estimated, against Japan, 1 ship sunk & one damaged for every 26 mines laid (IIRC; might have been as low as 1:52). Either way, mining could be very productive.
Almost as important, the losses to subs & aircraft on mining missions were very low.
Don't forget, too, the Brits (or whoever's the target of mining) can't assume mines aren't present: they have to sweep. That hazard, alone, can tie up traffic, & that's a headache: it effectively means lost shipping, even with no ships actually sunk. (The same applied to convoying.)
Actually not. Offensive minelaying is a well-accepted practise.
And the 3 day closure IMO is understated quite a bit. There was a famous U.S. case (which, I regret to say, I can't name...


) in which German U-boat mines closed the harbor for several weeks. I expect RN sweepers would be better--but they'd need lots more ships TTL...& what doesn't get built, in the meantime? How many corvettes get diverted from convoy duty to sweeping? (OTOH, does this encourage construction of new yards or repair facilities in Canada, especially in Newfoundland? Or encourage basing A/S Stirlings in NF, to take up the slack from 'vettes?)