The waters around the site of Cheonan's sinking has notoriously strong currents(3~5 knots.) Another factor going for them is the presence of many fishing boats in the area, which would make it easier for them to sneak in, as this article mentions.
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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.
You will find this very much of interest.