This sounds very likely. NK didn’t even need to do the mission on a given day against a given target. They could make several attempts on several targets of equal value, waiting for the best possible chance of success.
Since a large group of commandos is mentioned, probably there was no torpedo. Instead 6-8 swimmers were released near the SK ship. They approached the ship, installed explosives and left, to be picked up by the mini-sub. At some later time, when the sub was far enough (probably in NK waters, if not docked,) the explosives detonated.
In this case many swimmers would be needed to deliver a heavy, powerful explosive, and reliably (in darkness) connect the individual charges together for simultaneous detonation.
By the way, the currents are not always running in one direction. Some currents are reportedly change its direction at certain point in time. It was reported that the tide changed direction in less than 20 minutes of Cheonan’s explosion. I suppose it may have something to do with tide. The tide in this area is rather significant( some 30 feet difference between low and high tide. However, I am not an expert. I am just speculating.)
SK intelligence was aware of their activities mainly in east coast, but also suspected that similar operation is going on in the west coast.
So it is possible that one of their ambush teams got lucky and found 'a nice target.'
I want to understand the mission your envision where swimmers attached a large limpet mine to a ship underway.
Not likely .. the ship was reported as traveling a 12 knts ( per the skipper) at the time of the explosion.