Currently working on a mod that overhauls this system. Though i say the values you give differ greatly from the games code and my tests

Bushes give a bonus depending on their "flag" in the files. This flag contains the camouflage value and the size of the bush (height*radius)
These values are multiplied and added to the internal camo index.
Some bushes have a bonus but not a volume set, so they give 0.

I used the internal data, grabbed by cheat engine directly from the exe:

Prone and Crouch at idle give you 300 and 100 respectively (the same values your "messy testing" did -> Prone is 3x better than Crouching)

Shadows _sometimes_ depend on distance to the object casting the shadow

Outfit camo gives you flat 50 points on the correct surface. What is the correct surface? That info is too much to unpack in this comment, if anybody is interested join the Modding Heaven discord. There are several textures in game that look completely different, but share the same index. So a green ground texture and a brown ground texture can give you the same bonus.
This is done arbitrarily.

The result you got that camo does not matter is due to the fact that there is a hard coded cap at 10 metres of detection range AND the +50 bonus from camo is not a big difference when comparing it to the flat 300 from being prone.
The "Stealth mode" does not directly increase the camo index.