So the STL data comes from a computer generated model. What is the name of the software that generated these surfaces? What is the data structure of the surface within that software? Is STL the only outuput format? What other output formats are available, for example Parasolid or STEP?
As much as possible is not a tolerance, you must specify a number for the acceptable deviation. That might be a tolerance on the normal distance between the surface and the facet as I showed above. If you want to maintain sharp edges, you can specify a tolerance on the angle between the normal vector of the surface and the normal vector on the facet. For example, an angle of 30 degrees would show a lot of rounding while an angle of 3 degrees would show a very small amount of rounding of the edges.
As I mentioned in the video, increasing the number of samples increases the accuracy that the surface will follow the facet body. A perfectly sharp edge requires that there be two surfaces. This is what I did when I separated the top and bottom surfaces and created a perfectly sharp trailing edge.
To avoid reducing the dimension of air along the wing span by 0.2 mm, try making the air wider than the wing span and subtract the solid body using the Combine tool. Use the Pull tool to move each end face of the cavity out by 0.1 mm then you can split the body at exactly Z=0 and Z=30 mm.