Need answers to the "whys"

Those are all correct. And yes, I'd say they're being mentioned as one of many.

"As a final note" is a common expression, but I guess you could say the "final note" is one of many other "notes" that were made previously.

As to "the wrong key", it looks as if there's a definite key you shouldn't hit. The wrong key.

PieanneAs to "the wrong key", it looks as if there's a definite key you shouldn't hit. The wrong key.

Logic and English don't always go hand in hand. The is often used with wrong even if there are millions of wrong alternatives. A person answering the phone may say: "You've got the wrong number."

Cheers
CB

Thank you, all.

Would you say that if I dithering between the choice of whether to put "the" or to put "a" for the phrases below or the likes of them, a good thing to do is accept them as some kinds of idiomatic phrases and use them as such making some allowances for clear differences that might arise because of contextual diffferences?

I'm not sure I understand exactly what you're asking, but yes, for many expressions with "the" or "a", you just have to accept them as idiomatic phrases. The best way to get familiar with these little quirks of English is to just read a lot. Slowly but surely, you'll develop a better feel for the language -- it just takes practice and patience.

wrong number

Interesting, isn't it?

He never dials a wrong number. = Any number he dials is right. = Any number he dials is a/the right number.
He dialed the wrong number. = That/The number he dialed was wrong. = That number he dialed was a/the wrong number.
He never hits a wrong note.