[mothered]

First of all, my plan is to DNA both of the packages (according to Amazon, these arrive around the same day)

Are they arriving at the same time and from the same driver?

If not, It's not very likely that two packages didn't arrive from the one company at different times of the day.

Does it make any difference if I contact the third party company directly for the dashcam instead of Amazon? Or doesn't it matter, since I am reporting the entire package as DNA to Amazon?

If you ordered from Amazon, that's who you claim through.

If somehow the DNA fails -> Is the faulty item (for the consumable) and the defect item (for the cam, e.g. leaking battery / broken item) method a good option?

Are you planning to use two separate methods at the same time for each consignment?