Neil, I am completely stumped. I am not even remotely able to duplicate your experience, so I don't know what's up.
I assume that when you add your video to your timeline, you're accepting the program's offer to change the project settings to match the video specs. Or it might be worth going into the Project Settings (the cog on the Preview panel) and selecting the option to Match Media. But even then I don't know why you're seeing what you're seeing. Or why you're experiencing your other issues.
It might be worth uninstalling the entire program and even deleting the entire folder at C:Program Files/VEGAS/Movie Studio Platinum 16.0 and then trying a reinstalling using the files at https://www.vegascreativesoftware.com/us/downloads/ (Assuming you own version 16 Platinum.)
These files will be more up to date than the ones on your DVD and your serial numbers will still work. And maybe a scorched earth approach will help you get rid of these really odd issues.