As far as device placement is concerned, just avoid the problem and get one that's built into the windshield-mounted rear-view mirror, or mount the cameras outside the passenger compartment. CHP SUVs have them on their hoods.
As far as timing, the device I have can be set to record in files of lengths 1, 2, or 5 minutes. Unless the g-shock sensor is triggered, all recorded files will be overwritten.
I do not see how they can mandate no recording prior to 30 seconds before an event. We don't have crystal balls and events (i.e. collisions) are not usually scheduled in advance. Now, they can ask that recordings not be retained long-term for that many seconds before an event which is saved for some reason (e.g. insurance claim, court, etc.), but to deny recording in the first place nullifies the use of the recording system in the first place and is nonsensical - thus a law void on its face.
Keeping the first 30 seconds after an event and not longer (unless another event occurs) can be mandated.
Note also the definition of "video event recorder" - that it EXCLUDES devices which continuously SAVE its record (CVC
26708(a)(13)(A)). So, how does one "record" without "saving" the data? This is what happens when politicians write stupid laws.