Just rushed through all the posts and although the videos do look awesome on the HDR TV connected to a laptop, I don't think it was mentioned that the Viofo A139 Pro doesn't record HDR content at all?
As JardaB says, bit depth and HDR are different things, you can record HDR in 8 bit, 10 bit, or 12 bit. 12 bit is often used for professional video cameras so that there is enough detail for editing and still have good 10 bit colour resolution, 10 bit is the standard for HDR consumer products such as HDR TVs, and 8 bit is rarely used for HDR but is the standard for SDR. The bit depth is the colour resolution, not the dynamic range.
It merges two pictures with different exposure into a single SDR picture and calls it "HDR", since it captures a higher dynamic range. But it doesn't really have anything to do with capturing a single picture directly with 10 or 12 bit depth.
Merging multiple exposures into a single frame also is not HDR, multiple exposure frames can be done with SDR or HDR. In dashcams the aim of having multiple exposures is normally to record a larger dynamic range than can be recorded with a single exposure, with the added advantage that the top part of the dynamic range has less motion blur than it would with. a single exposure. Note that the A139 Pro can easily record a large dynamic range without using multiple exposures, so both HDR On and HDR Off settings do produce a much larger dynamic range image than fits into the official definition of SDR. Also, it is not necessary for a multi-exposure frame to have a larger dynamic range than a single exposure, you may use the multi-exposure purely to reduce motion blur in the top part of the range.
Dynamic range is measured in nits, 1 nit being 1 candlepower, 100 nits being the brightness of a standard SDR cathode ray tube (CRT) display, 1000 nits being the brightness of a good quality modern LCD screen, 16000 nits being the brightness of the sun. The A139 Pro always overexposes the sun, often over exposes the brightest parts of clouds, oncoming headlamps at night tend to be at maximum exposure rather than overexposed, so the A139 Pro has approximately a 1000 nit range - the standard HDR range for HDR TVs. It certainly records a range far larger than can be displayed on an old CRT screen, the standard for SDR, so yes, it does record in HDR all the time, and if you turn the HDR setting On then you get "Multiple-Exposure HDR", the name of which doesn't fit in the menu so is shortened to just "HDR"!
And I'm not sure if there's enough detail in a 8-bit "merge" picture to look similar to a single frame 10/12-bit picture, when being stretched out.
There is plenty of detail for dashcam use, it can look a little pixelated when converted from 8 to 10 bit if displayed on good equipment in good lighting, your eyes can't see the difference between 10 and 12 bit anyway. The only problem I see with copying an 8 bit file into a 10 bit file is that when it is then compressed by Youtube, Youtube sees the pixelation as an actual image feature, which can result in compressed Youtube video being significantly more blocky than necessary. It would be better to upload it to Youtube as an 8 bit HDR file, but I have never managed to get Youtube to accept such a file, it seems to like HDR10 files.
A bit similar to standing in the middle of an orchestra in an awesome concert hall, then recording it with a stereo microphone and then running a surround sound generating tool over the recording, which tries to interpret the different reverbs and sound shapes due to the different direction the instruments came from and places them onto the different speakers of a 7.2 setup.
The A139 Pro is recording HDR brightness levels, at 8 bit colour resolution, it is HDR. It then stores the video in a file marked as an SDR file, which is definitely incorrect, but it does make it playable and editable on everybody's video players and editors, there would be lots of problems if it didn't. In future I'm sure we will see dashcams that record proper HDR files, with very similar content, maybe with 10 bit colour resolution which would be nice for both road trip movies and compatibility, but for dashcam use 10 bit is not actually needed and a bit of a waste of space. The important thing for recording sunny days and reflective licence plates lit by bright headlights at night is the high dynamic range, and we actually already have that in the A139 Pro.
and although the videos do look awesome on the HDR TV connected to a laptop,
Yes, that is how they should look, a sunny day with bright white clouds in the mountains should look awesome, and far far brighter than you can display on an old CRT screen that is the definition of SDR. The A139 really does record High Dynamic Range video, and it is best viewed on a 1000 nit HDR screen. The process of actually viewing it in HDR is a bit more complex than it should be, but it is worth doing.
... or are those heavily edited as well?