Unfortunately, this is not even stated by the manufacturer: http://test.aukey.com/products/dual-port-quick-charge-3-0-car-charger-cc-t8 I guess the standard length is 1 metre.

I've been using it with my Iphone and dashcam and it charges quickly and has a nice design. but in the end it all depends on the state of the car battery

Unfortunately no :(

Yes, of course! :) It would simply support up to Quick Charge 3.0 :) It simply charges your Quick Charge 2.0 device as fast as with the included charger from the manufacturer :) so with 2.0A or 1.87A

The Aukey charging adapter has two retaining springs on the side. That should normally hold it. I think your cigarette lighter is broken if you have already tried 4 charging adapters. Have you tried the other charging adapters in other vehicles? Do you drive an off-road vehicle? Is the cigarette lighter mounted on the ceiling?

Yes

Yes, the adapters recognise the maximum charging speed of the connected device and adjust accordingly. However, this product comes with a micro-USB charging cable that will not fit the iPhone.

For the iPhone X we recommend the following model. Dodocool DA135 3-coil (USB Fullsize, 1.20A, Qi compatible) This supports "Fast Charge" 100% and is even QI compatible.