The coelocanth that was found was indeed very different from the fossil ones. It was a different species and a different genus; it merely belonged to an order which had been thought extinct.
But if time and mutation are the inevitable drivers of evolution and if these processes are constantly at work changing the species, then how can you explain the lack of any significant change in the Celocanth over a period of 100 million years?
You are forgetting the other part of Dawin's theory-- natural selection. In the absence of any environmental pressure to change, time and random mutations will not produce dramatic changes. That is why cockroaches have not changed all that much in the fossil record. The coelocanth, living in deep oceans, had indeed changed, but not as much as creatures who lived in environments that had changed more.
I will check out your first point. But your second point explains nothing. There were presumably many species swimming in the same sea as the Celocanth. And yet only the Celocanth displays no significant change over a 100 million years? Nice try but it doesn't explain the data.