Right - So ....
1 - as I explained above, 64-bit Linux doesn't / can't restore old device ID's at this point - you would need to log this as an enhancement request.
2 - Assuming that your devices DON'T change names upon being re-imaged, then using "MAC and NAME" is fine. If they do change hostname upon the OS rebuild, you may want to switch to MAC-only / or a MAC OR HOSTNAME logic. That's what I tried to explain at length in the article.
The actual Maintenance runs (by default) at 23:00 each day.and you can (as of 9.5) even enable a log of the purged devices in the ADVANCED settings for the Inventory Service (enabling this / changing Inventory options usually requires a restart of the Inventory Service to take hold).
Does this clarify things?