1.) How do you deal with this part of the User Interface?
As with most spammy information it eventually becomes "white noise" to me and I just filter it out. It's not particularly jarring to my experience. I don't really think about it much.
2.) Why do you think it is absolutely necessary to be informed, in the middle of the screen where you are, when your retainers have returned, and every other redundant thing that gets placed directly in the middle of the screen?
It is not.
3.) Why do you think having the option to turn this off would absolutely ruin the game and its 100% sensible, that Square-Enix has refused to add this option 7+ years into the games life.
.. I don't.
Are YOU aware of what a loaded question is?
4.) How does this constant flood of useless information not irritate you?
See 1.
5.) Do you enjoy the sound of a fly constantly buzzing in your ear?
No.
I see your problem, I don't argue that all UI elements and notifications should be toggle-able. I also don't know why you come off on your first post swinging at the reader. I don't have a particular reason that they haven't added the option yet, but I also will note from a lot of the responses that most people have just dealt with it as part of the game and there doesn't seem to be a pressing need for it.