glojo, the safest place for a Kilo is motors stopped, neutral buoyancy at 500 feet beneath several saline layers, can't be detected but can listen. The most dangerous place for a Kilo is in shallow, restricted waters, can be detected (perhaps even visually if the water is clear) but cannot hide at 500 feet beneath saline layers. With only one Kilo, the likelihood of a carrier sailing conveniently into a firing solution in the Gulf (as opposed to the Strait) is remote, hence the more effective use of 12 Kilos strung out in a picket line (history, repeat, etc). Re: mines, you're moving the goalposts! I've considered torpedo attack only. A Kilo is an expensive asset for mine laying.
It is plain wrong to suggest Iran does not have the ability to sink a battle group
A battle group is considerably more than a carrier, but - for the Iranians - it's the carrier that needs to be the target. The lone Kilo's biggest problem is its relative lack of submerged speed, cannot stalk the carrier and get behind the screen. A working Alfa boat would cause a few headaches. And then it would stop working.