‘scholar’:
My question is that if this 70-year period of fasting began with a definite event such as the destruction of Jerusalem then what event marked its ending in 518 BCE?
The ‘event’ was that it was then that Sharezer and Regem-Melek asked the question. It is directly stated in the passage. Some people really are thick. Your question implies that you think that some remarkable things should happen beyond simply asking about the fasts, probably for some weird religious symbolism or some other superstitious ‘significance’. 🤦♂️