@Stefan
My friend is overwhelmed with work so just some speed advices! ;)

Be careful when you make bas relief, in general you prepare the model that will serve as a support for the projection accordingly, i.e. be careful that it is as much as possible in a single plane.
It is not impossible that the artist applied some transformations, see dynameshed his model just before making his projection.
Moreover I admit that I will tend to make a projection of SubTool and eventually use the brush ZProject to eventually make touch-ups.

It should not be forgotten that ZProject also works with ALT depending on the position of the model and the projection...

Hope this can help a few...