[kreativasas]

I have used Simplify 3D & Ultimaker Cura 4.9.1 both have plugins (Mesh Tools) that can make an asset Watertight, some are paid for some are free.
There is also this mikedh/trimesh: Python library for loading and using triangular meshes. (github.com) but i have never used it myself.
As far as merging meshes is concerned i have never tried it with powertoys, i use Sketchup to make my files for 3d printing (as a plugin that will prepare and make sure mesh is watertight).
If you know python the GitHub repo maybe your best option, you could convert the code and make a DLL for your application (depending on the licence of course)

Unfortunately I don't know Python.... but i need to do it runtime. Is there a way to do it with powertoys, or maybe is there a lib for visual studio ?