@Keet
0. Thanks again! As a non-programmer, it's an honor.
1. There are still many improvements to be made.
2. I have already installed Blender, but I also have AOI (ArtOfIllusion) .
3. Thank you for the advice! I have some knowledge of C++ (using QT) and Python as I know few things in Basic Spectrum.
4. Your suggestions are good, and I have been considering them for a long time.
4.1. Regarding smoothing, I believe there are some parameters in the obj file. I have seen that AOI imports such things.
4.2. Regarding bending a cylinder, I believe it is easier(for me) to create a bent cylinder directly by extruding (like under AOI).
4.2.1. I think I'll add "Extrude room" as other action of this plugin.
5.

<<I wanted a completely flat surface with a cutout.>>

This plugin works with volumes. First, it checks whether the two volumes overlap. 2*3*0=0

<<Now I just have to remove all the faces below the top surface with Blender.>>

I'm glad you found a workaround.
It is fool of bugs so, keep test it. Eventually test the others actions also.