The double click did not work for me and I had to use the 'option' and GO to get library/Application support. That folder you refer to was not there. So I was back to my original issue of it saying 'a newer version exists'.....
Which double click? System disk, Library, or Application Support? The "Home" folder in the sidebar is single click. Since you are using Sierra the Option/go combination for the Library folder is no longer valid.
I assumed then that the link to 1340 in 'patches and updates' wasn't the full install but just updates? (I am probably wrong) !
The link in "Patches & Updates" is a full version. I used that same link to get my copy.
So do you think it was the plist files in Library-preferences all the time ??
No, plist files just support the app with startup info. The preferences folder did not exist in El Capitan so this must be something new in Sierras' file structure, at any rate, you need to find the "com.imsi.turbocadmacprov10ESD" folder and remove it. It should be in the Application Support folder, perhaps you overlooked it when searching.
After installing TurboCAD, the first time you launch the app it builds the folder "com.imsi.turbocadmacpro10ESD" if it does not exist. If the folder does exist it skips this step. If this folder exists because you had an older V10 version installed then you are now running a newer application with older support files and this may be the cause of your current problems.
I now try my 'black line issue' using an 'image' as a decal on a covered surface but when I 'Render to file' it says DEMO MODE
Can you just render to the screen without black lines? What settings are you using for Render To File?