My car is 2012 and I also had problems with the aluminium rivets coming loose. I spoke to the factory about this and they sent me some 16.5mm stainless steel ones to replace them.
I found that the three rearmost rivets on each side do need to be at least 16.5mm long as they hold the body to the chassis rail. The rivets forward of these should be 9.3mm. That's on my car anyway - yours may be different!
The stainless rivets are hard to fit and you will need a long handled heavy duty tool such as
this. I also placed a wavey spring washer under each rivet head to take up any slight movement in the future.