.... I think the seller on Amazon may be misrepresenting what he is selling. Sorry I missed that detail last night.
I would recommend returning the lens and getting the 5MP 6mm-22mm lens. It is overall a much sharper and better lens with greater useful range than the 3MP.
It may or may not work with your current lens holder. I think you may still need to obtain a longer lens holder. Either way, here's what you need to know.
With the measurements you have already made it sounds like you are on the right track.
As you know, you need to have the rear element of the lens at the proper distance from the sensor to achieve focus. Earlier, I suggested that this should be pretty easy because a varifocal lens can adjust for this requirement unlike a fixed focal length lens like what we usually have in our dash cams. For example, the back focal length range on the M14 version of the 5MP 2.8mm-12mm lens is listed as between 7.15-12.75mm depending on how you set the focus. Obviously, if you can't get the rear element somewhere in the range where it will focus, it won't work. So, the specification you need to be aware of is called
BFL (back focal length) This is the
optical focal distance between the rear of the lens and the sensor. The other relevant spec is the
MBF (minimum back focus) which is the
physical distance between the rear element of the lens and the sensor. Not every vendor lists these specs completely or at all. Most don't seem to mention MBF but BFL should tell you what you need to know.
The optical BFL for the 5MP 6mm-22mm lens is
listed as 6.4mm. (no range is listed) The BFL for the 3MP 2.8-12mm varifocal lens you apparently purchased is
listed as 6.39mm, so virtually identical. Again, this suggests that you may still have problems unless you get a longer lens holder or make some other modification.