Receive an alert email/Mobile phone once a motion is detected at specific time (i.e., after midnight).
Yes using motion alerts with schedules.
Set a custom retention time (i.e., for 150 days).
https://documentation.meraki.com/MV/Initial_Configuration/Video_Retention#Setting_a_maximum_retentio....
You can set up to 90 days, note depending on the MV capacity, quality & resolution configured, as well as other settings the actual amount of footage that can be retained with vary.
As @alemabrahao mentioned, documentation.meraki.com is the best place to search for these sorts of questions.