Tris to Quads only pairs triangles into quads, and on CAD meshes it often leaves fans of triangles on big flat faces or on faces with many corners. STL CAD Cleaner joins every coplanar triangle of a flat face into one n-gon (a 48-vertex cap becomes one face), with a distance test so curved surfaces are not flattened.

No. Joining coplanar triangles does not move any surface. The only tolerances are Distance (0.01 mm by default, for welding) and Flatness (0.01 mm), and the report shows the volume before and after for closed meshes: +0.000% on clean CAD parts.

Blender faces cannot have holes, so the flat top of a plate with holes can never be a single face. It becomes a few n-gons around the holes instead of hundreds of triangles.

STL files have no units and Blender's default scene counts 1 unit as 1 metre. Set 1 Blender unit = to 1 mm in the panel so Distance and Flatness are real millimetres.

No. It is for editability: welding, flat faces, sharp edges, shading and origin. If the mesh has holes it is still cleaned and the report says it is not watertight. Print Fix, our repair add-on, is the tool for holes.

Yes. Preview Result is a dry run that reports the face count, n-gons, sharp edges and volume you would get without changing anything. Ctrl+Z also undoes a clean in one step.