i dunno who gave isshin the charm but he was trying to make ichigo hold on to it, well he probally wanted to give it to Ichigo so when Kon was in his body then he'd be able to sense him, cause Isshin said to Kon that he knew what was going on from the very start, like when the went to soul society.
They hafta explain how he somehow lost his shigami powers cause he said he was powerless to save his wife at "that" time, or somethign along the lines of that.