I have not yet synched my google calendar on my iPhone to my M3. If I do, how secure is this calendar info? Once the M3 has my calendar data, can it be accessed by others (Tesla, etc.) through the LTE connection?
Arguably it can be accessed, but so can your cars location, charge status, driving history etc. The Tesla app has access to your calendar, location, contacts, and local files. There's a balance between security and usability with all of this, so it depends what you're comfortable with allowing.