They say « Better late than never », but still I’m sorry for the long delay in getting back to you. It took me a while to solve my problem but still I thought it could be of interest to the other forum members for me to share the outcome.
To reopen my corrupted VSP files, I had to reload my PC with dual boot (Windows 10 and Windows 7) which was quite a feat for me. I then succeeded under Windows 7 in opening my corrupted VSP files, extract the culprit (.c3D files), replace them with MS3D output video files, then render.
Nevertheless, to retain the overlay transparency of the .c3D files, thanks to your advice Ken, I had to produce AVI files using MS3D File>Export Video Overlay .AVI files, insert them into the VSP, and everything worked fine, even when I switched back to Windows 10.
So my conclusions are as follows:
(1) MS3D produces gorgeous 3D effects but is very complex and not obvious to use (very time-consuming)
(2) Per Corel technical support, MS3D is not supported under Windows 10 and can bog.
(3) Inserting MS3D .c3D files into a VSP file under Windows 10 can make VS crash (message ”VS stopped working...”) and could corrupt the VSP file
(4) MS3D seems ok under Windows 10 if, per Ken’s advice, we produce MS3D .AVI overlays to insert into a VSP file instead of a .c3D file. Also, I cannot say if that contributed or not but I upgraded from VSX8 (which was designed under Windows7-8 era, I believe) to VSX9 which was designed under Windows 10 era ???
(5) However MS3D .AVI overlays are huge files even for a short 10 seconds 3D effect (we are talking gigabytes); if resulting MS3D .AVI overlays are to be kept for future rework or upscale of the VSP file, the overall project package will be huge (alternative is to keep only the .c3D corresponding file in archive and rebuild a MS3D .AVI file later if ever needed, as long as MS3D still works or exists then).
(6) Also scrubbing with a huge .AVI file like that results in jerky preview and tougher synchronization.
I still use MS3D under W10 with VSX9 and it works OK up to now. If anyone has suggestions on how to better use MS3D under Windows 10 or can suggest an alternate/equivalent 3D program usable with VS, your input will be greatly appreciated.