Perhaps I haven’t understood what you are proposing, but it seems to me that the scheme described above is completetely equivalent to just setting the Default URI Match detection to “Base Domain”.
Suppose that your active browser tab is displaying a form located at https://webmail.domain.com/login/, and you have a vault entry set up as described in your post. For all three of your stored URIs, Exact matching will fail ⇒ Host matching will fail ⇒ Base Domain matching will succeed.
Suppose instead that your active browser tab is displaying a form located at https://training.domain.com, and you have a vault entry set up as described in your post. For URIs (a) & (b), Exact matching will fail ⇒ Host matching will fail ⇒ Base Domain matching will succeed. For URI (c), Exact matching will fail (unless you add the https:// protocol to the URI) ⇒ Host matching will succeed; however, if you had just set the Default URI Match detection to “Base Domain”, this would also have allowed URI (c) to match.
Can you come up with a scenario in which your “preference order” scheme would yield a different outcome than just setting the Default URI Match detection to “Base Domain”?