As to the height you may want to consider the use of a string pot, yo yo pot or draw wire sensor (they have a dozen names ). What I am getting at can be seen here. They are marketed by several companies. Looking at your initial pictures they may fit into the scheme of things as to an accurate indication of the height of the dropping mechanism above the dash pot. A relatively small one with an analog output would likely work well for your application.
As to the release something you need to note is the signal you use for a start. Since this operation happens pretty quick the start and stop signals need to be pretty accurate. For example if a relay controls the magnet just looking at or using the relay contacts to drive something to get your signal may lie to you. Relay contact open times need considered when dealing with milliseconds. I believe using a hall effect device is the best way to go about it as it should provide the most accurate start time.
Ron
As to the release something you need to note is the signal you use for a start. Since this operation happens pretty quick the start and stop signals need to be pretty accurate. For example if a relay controls the magnet just looking at or using the relay contacts to drive something to get your signal may lie to you. Relay contact open times need considered when dealing with milliseconds. I believe using a hall effect device is the best way to go about it as it should provide the most accurate start time.
Ron