1. Auto import appears to destroy tags in DVDs always.
This doesn't happen to me. In my experience Auto-import rereads the file contents and replaces certain field values with the default values only if the file path has changed (in this case the VIDEO_TS folder contents are imported as a new library entry) or if the last modification date has changed (when "Update for external changes" is enabled).
... b) They would have to somehow be "saved" and imported into the other MC databases.
It appears to me that there just isn't a good system for storing tag information in DVD files. Most standalone media files have great tagging abilities. Standalone files are great at tagging and DVDs are great at menus, extra audio/video tracks, subtitles, etc.
It isn't just DVD Videos. Only a few video container file types can store file tags and currently MC supports only WMV tag writing.
I wonder if you have forgotten the MPL system already.
You can save all library fields in an MPL file. For example: select the file folder in MC tree > Video > Disk Location and do File > Export Playlist... (select mpl in the options)
You can apply the tag values back to a library file (or files) simply by importing the MPL with the File > Import Playlist... tool (select "Common playlist types" in the "Files of type" box).
I always create an mpl file after I have tagged a bunch or videos. Normally I store one mpl in each "movie" folder or if the folder contains several video items like recorded TV series episodes I store an MPL that contains them all. When I save a folder specific MPL file I use the "relative paths" option and the file will work from any location if it is kept in the same folder with the video files or VIDEO_TS subfolder. This makes possible to restore the tags after moving a video folder outside MC.
In addition, I periodically save an MPL file which contains all video files in the root of my video location.
EDITFeature request: a "sidecar file" option for untaggable file types.
It would be great if MC could automatically create and maintain an MPL file in the file folder. It would then also update the file when tags are changed.
MC could have an option for doing this whenever the file type is known to be not able to hold tags.