It sounds like you have the hard drive health issue covered. Hard Disk Sentinel pretty much uses the same SMART data, with a little bit of extra checking, I think. Best for individual drives rather than NAS drives in an array. I'm not sure it will even run against a NAS in an array, although they probably have a version that will.
You can view files that MC thinks are missing on any View or Smartlist just by entering this expression in the search field: [=ismissing([filename])]=1
You can also enter that expression as a "Custom" rule in any View to limit the View to only show missing files. Copy the Audio "Files" View (drag the Files rule up to the Audio heading, drop and select Copy) and add that rule, then you can check whenever the situation occurs, as long as MC hasn't re-imported the files yet.
You can also see visually in any View that has the "image (small thumbnail)" column. Missing files have a red circle with a white horizontal bar through it, as per the attached image.
I can't think of any reason other than the file going missing for the [Date Imported] changing. It is a system managed, non-editable field. It only gets set when a file is imported. Of course, if you have accidentally removed files from the Library, perhaps when you were deliberately removing some files, and the file selection included more than you thought, that would do it.
There is another setting in Auto Import, "Ignore files previously removed from library". If that was unchecked, files that went missing and were removed from the Library manually or via "Fix Broken Links" would be re-imported when they are found later. Perhaps tick that checkbox setting now. Then if you run Auto Import manually MC won't re-import files that went missing and you may be able to identify what is going on.
It would still be worth searching the forum for discussion on [Date Imported] as there have been a lot of discussions about it, updating it, why you can't, etc. Maybe there is a hint in those threads that I can't remember.