I definitely agree with the first 2, the 3rd I don't understand, the "a" tag could be added on subsequent (NIP-22) comments to the URL bookmark, and then it will include the "d" value, i.e. the URL.
Discussion
That a tag always is confusing me..
For example when I follow NIP-23, kind 30023 events can have an a-tag too correct? If so, why not for other addressable events?
They don't have an a-tag generally, no. Only other events that refer to them.