Converting from one system or methodology to another is always a difficult and sometimes a big task. Especially for users. I would first set up MC views that met all their needs, and then I would take away the crutch they are using. i.e. Make it so that they can't run Windows Explorer, so can't access files that way. Tough love though. There will be more complaints. They will learn.
In all my years of doing change management the four things that most threatened a project were the requirements not being defined correctly, the solution not meeting real user requirements, training, and leaving the old system in place for users to use as a crutch.
If your family are all using MC Clients, and the MC Server is a separate PC as seems likely from what you have said, you can give the Client Read Only access to the MC Server, which will also stop accidents. You should probably also ensure that they are Normal Windows users and not Admin users on the Client PCs. I would then change the files back to be read/write, so that MC can do its job properly. If you wished you could even set up permissions so that your UserID could write to the files, but their UserID could only read them. But that may be a level of complexity you don't want to get into. I haven't looked into the Users function in MC, but maybe that can be used to restrict what your family can do so that they can't break anything. But if they have Windows Explorer, they will fall back to using it, and will continue to break things.
The video_ts.ifo files contain configuration information like menus, subtitles, aspect ratio, languages, etc., so probably not a good idea to delete them if you ever want to use the DVD Menu instead of just watching the main feature. MC can use the DVD menus, and sometimes they are handy for extras, languages, and other features. BTW my DVDs that have been imported into MC all have file names that end in video_ts.dvd;1. That is, I believe, internal MC nomenclature which points to the main feature in the DVD. I don't know if it points to the BUP, VOB, or IFO file, but if you deleted the IFO files I'm sure MC would still import the DVD and allow tooltips. If you did delete the IFO files you may need to reimport the DVDs though. But I wouldn't delete the IFO files. I'd fix the real core issue.
Anyway, MC is going to put not only jpg files, but JRSidecar.xml and possibly subtitle files next to the movie file. So it really isn't a good idea to launch a movie using Windows Explorer. Once that issue is resolved, there is no longer an issue with the image files.
On the bright side if you show the family the power of MC Theatre View, to search for, sort, and categorise movies and other media files, I think you can win them away from using Windows Explorer.
Want to watch a Horror film? Here they all are.
Want to watch something from the '80's? Here they are.
How about all the movies that Clint Eastwood made? Here is the list.
Movies recently added? Here you go.
Particular director? Yep.
HD only? Here is the list.
Want some other view? That can be built as well.
You should be able to knock their socks off.
Another point: You don't need to update directory structures to use the full power of MC. You just need to have good tagging of the files. The "Get Movie & TV Info..." function does that for you. Sure, it would take a while to do 8000 movies, but you could easily do them 100 or 1000 at a time, once you understand how that works and what to expect if movies aren't well named. You can update directory structures and files names later, if you wished. Or not. Once you have the tags in place, or enough to experiment with, you can build Views that the family wants.
Finally, I can understand your point about using thumbnails instead of image files, particularly on MC Client PCs. But that isn't addressing the core problem. If your family don't have a jpg file to accidentally click on, they will click on the xml file. Or something else. Use the power of MC to show them a better way.
How does all that sound?
