Trimming the extension tube would not help. The extension tube threads inside the existing mount tube, then expands out for the threaded portion of the extension. This expansion takes a non-zero length of tube. Since the new lens tends to focus around 0.5mm beyond the end of the original mount, this falls right in the middle of the area that the extension tube needs to transition from the part that goes into the existing tube and the extended part of the tube. The only way to use an extension tube is to move that transition away from where the lens needs to be, and the only way to do that would be to trim the original mount which I can't replace and don't wish to modify.
Screws are scheduled to arrive on Tuesday, I will be able to continue work on the spacer then.
I don't know what it is you are trying to say here. It doesn't make sense. And what the hell is a "non-zero length of tube"? I get the impression you don't understand how these lens extensions function other than the fact that you've seen some photos of them. Have you actually tried it? Since you rejected the suggestion to purchase a lens extension, or modify one, apparently not, so you are merely theorizing from the sound of it.
A week ago, you didn't even know what an M12 lens was and now you're explaining to us what will or won't work for properly mounting one and getting it to focus properly? As someone who has been modifying dash and action cams with alternative lenses for almost ten years I'd need to see a more plausible explanation and / or demonstration for your theory of why a lens extension won't work. Indeed, it's not exactly clear what the issue is with the exisitng lens holder. Are you saying that the threaded lens tube is too short or too long when you say that
"the new lens tends to focus around 0.5mm beyond the end of the original mount"?
In any event, the real problem here is that you didn't buy an appropriate lens for your project in the first place. If you had, you wouldn't be having to go to all this trouble attempting to get it to work.
To make matters worse, the vendor you purchased from is not a lens specialist, but instead sells general random merchandise and parts and they just happen to have a selection of M12 lenses in stock. The result is they don't provide any of the necessary specifications for a knowledgeable buyer to make an informed decision. They even describe the lens as "wide angle", "panoramic" and "macro", none of which are correct. (It is in fact a fairly significant telephoto but they don't mention that at all). They also describe the lens as designed for a 1/3" sensor format while also describing it as 1/2.5" format. (which it is). So, aside from the lens not having a suitable BFL for your camera's module the lens is designed for a sensor with a diagonal measurement of 7.19 mm. The sensor in your camera has a diagonal measurement of 6.46 mm so the result here is that the already high magnification telephoto lens you purchased will behave like an even longer focal length telephoto lens which in my opinion is overkill and an impractical EFL for what you're trying to accomplish with this dash cam mod.
I would suggest returning the lens if possible and perhaps purchasing the
8mm lens they also sell. It will provide a more suitable AOV, be less prone to vibrations in your images and you might even get lucky and receive a lens with a viable BFL. You might also ask the vendor before purchasing and see if they can provide the information you need about what sort of lens holder your new lens requires.
I would also suggest installing an
IR-cut filter on your lens, whichever one you use. If you do you will need a
UV cement pen to properly and safely mount it on your lens.
Typically a
good vendor specializing in lenses will provide all the relevant specifications for the lens you are purchasing so you know what you are buying.