Hello, I'll try to elaborate the scenarios you mention:
1- i guess in that case the best option would be to re-download all notes from Evernote Cloud, but if you can't for some reason then you should import latest full backup, then latest incremental backups as needed. For example, I backup a monthly full, weekly and daily so I would need to import full+at most 4 weekly's+at most 7 daily's = 12 imports in the worst case for each notebook.
If you'd like to make this easier, then you can full backup every day, and then just 1 import for each notebook. Or maybe build a script that imports and sorts everything in place.
Note that in any case, you'll lose the last day worth of your notes!
2- Similar to 1
3- Same thing, then find the note in the imported notes notebook and retrieve the attachments.
4. Same thing as 3. You can import several full backups and then use Evernote search function to find it.
It would be pretty easy to modify the script so as to export a single enex file for every note. Would tha be more useful? Take into consideration that HTML files do not preserve tags for example.
I am aware of Evernote Note History, I recently had to use it in order to retrieve some attachments which got deleted in the latest big bug.