I agree with everything you say about politeness for tablemates. Getting to know your tablemates is standard cruise ship protocol and is a part of traditional cruising.
Unfortunately, cruising is not nearly as traditional as it used to be and I think even the Maitre D's have decided that things have changed.
I have been in your shoes so I know exactly what you are saying, I also dislike dining alone.
Unfortunately I truly believe society has changed in the last ten years or so. I think the vast majority of people feel they are not responsible for other people - especially strangers (before the cruise) and they don't feel that just because you are tablemates it means they have any obligation to notify you of anything.
In a way, even I see the logic in that. I am not going to get dressed for dinner if I have just awakend from the kind of nap that leaves you groggy and feeling non-social. Often that is just 20 minutes before dinner.
So, unfortunately the single is more on her own now.
Honestly, I would recommend switching to line that has single cabins (Norwegian Epic) or Holland America which at least does still have a singles matching program and social hosts onboard.
Another line with single cabins is the new Voyages to Antiquity. No singles supplement.
Or, as I said, I would really think about switching to anytime dining, because that way you can walk in and ask to be seated at a table with other diners, and there is no relying on your assigned tablemates night after night.
I have been in your shoes, believe me. I was on an MSC ship where I was a solo American on a ship with only one other American (also a travel writer). He ended up being kind of a jerk, because we had agreed to meet for dinner every night until one night when I literally got stuck in Mallorca and could not make it back to the ship. I called his cabin when I got back and left a message explaining what happened and apologized, but i think he decided to ignore me for the rest of the cruise after that because he claimed he never got any of my calls asking him to meet me for sunsequent dinners - so I was alone the rest of the cruise.
Until it happens to you - you don't realize how un-fun it is. That is why I seldom recommend to singles that they cruise alone. I have personally "been there" and didn't enjoy it.
I also meet single cruisers who want to cruise so badly they will make that leap of faith every time and believe they will make friends onboard. When it happens - great. But when it doesn't either you have to get used to plenty of quiet time or else it is a bummer.