Hi Jim,
1.) There is no “CurveThoughtPt” functionality in openNURBS. If you search the Internet, you can probably find several papers that describe the process.
2.) The code behind the “PlanarSrf” command is not in openNURBS. But you might get what you need by using ON_BrepTrimmedPlane. See opennurbs_brep.h for details.
3.) OpenNURBS will not create meshes from surfaces or polysurfaces.
Keep in mind that openNURBS is designed to read and write Rhino files. It is not a general purpose geometry library.