Should I sign an inventory of fixtures form and how does it relate to the insurance?

Make sure to fill out the rental contract before every departure and upon every return of the vehicle. 

In the event of an accident, it is the only document that can be used as proof for the insurance. Without this document signed by both parties, the insurance will not be valid.

This is available before every departure from your profile: My Bookings > click on the relevant booking > Review my Booking.

Make sure to print this off in advance and to allow enough time for this to be filled out before departure and upon return.