So I got myself a server for further testing and to see the trajectory as shown by the game's built-in trajectory render and this is what I found.
I repeated the shots from my first and second post as that is a nice spot to shoot from.
First some notes, the point of origin for the bullet's trajectory is the crosshair, this is why on this following image, the point of origin is a little bit above, to the right and a bit behind the character. The angle at which I took the screenshot make it look more behind the character than it actualy is.
As the image shows, my theories from previous two post are proven wrong. The trajectories and points I have indicated on the image may be a little bit off but I think they speak for themselves.
As I noted previously, the bullet's trajectory originate in the crosshair, given that the crosshair is in a different spot, depending on the player's gameplay perspective a different trajectory will be made even when shooting from the same spot with aiming at the same target. Please note, that the trajectories goes differently due to the spread, but the point of origin of the shots is visibly different.
Given this realisation, I went to test a potentional exploit and found it to be existing. In the following picture, you will see the shooting player being hidden behind a wall, with the musket clearly pointing into the wall, yet still being able to shoot at targets behind the wall.
As previously noted, given that the trajectory's point of origin is different between FPS and TPS, a first person player will be at a disadvantage. Please note, that in the following screenshot, the shooter is standing in the same place and aiming at the same spot as in the TPS picture. The shot got stopped by the wall.
As a conclusion these observations were made quickly, so there may be some margins of error when taking shots or when taking screenshots as it is not easy to old the right mouse button, the freelook key and pressing F12 to make screenshots.