The Good, the Baad, and the Translation of Psalm 23

I need some help here. Aristotle didn't translate, and he didn't use the word ψαλμὸς. But consistently, those who knew them well rendered תהילים (i.e., Tehilim, or the "praises") as Ψαλμοί (i.e., the Psalms, or the "strummings"). And today, Mr. Benyamin Pilant, who maintains the copyright for the Jewish Publication Society's English language TEHILIM, calls them the Book of Psalms.

The translation of this word seems good, and since the Septuagint it has seemed good enough.

But some time after the translation of the Jewish scriptures of all sorts into Greek, problems ensued. And now we all have a few questions in English. What answers can we find?

1. The disciple Ιωάννης (i.e., John or יָנִיב?) translated the Hebrew Aramaic words of his רַב (i.e., his rabbi, his teacher) into Greek this way:

Ἐγώ εἰμι ὁ ποιμὴν ὁ καλός
(or, in English: "I am the good shepherd")

But why?

Was John's a good translation or was it (as sheep might translate that) a baaad one?

2. Was Rabbi Joshua (i.e., יְהוֹשֻׁעַ or Ἰησους or Jesus) paraphrasing David's praise to God, when the shepherd boy after God's heart was singing this?

יְהוָה רֹעִי
(or "HaShem is my shepherd" / "'I am' / 'He is' / 'The Name' is my shepherd")?

And if Jesus adds "good" when it's not in the original is that baad (for sheep)?

3. Or was John looking at the Septuagint (as he seems to do when he begins his gospel of Jesus, giving a nod to the LXX beginning of Genesis)?

In other words, did John add "good" or ὁ καλός when translating what Jesus said?

And, if John was reading the LXX, is it baad that the Jewish translators (from Hebrew into Greek) lose "good" in the last verse of Psalm 23:

טוֹב וָחֶסֶד

becomes

καὶ τὸ ἔλεός σου

(or "Surely goodness and mercy"

becomes

"and your mercy")

And when losing "goodness," the baad Greek gains "you," a very personal (vocative, 2nd person personal pronoun) reference to "master my shepherd" that the Hebrew has lost in verse 6. Why?

4. What if John and his rav Jesus (or יָנִיב and יְהוֹשֻׁעַ / or Ιωάννης and Ἰησους) could read David's תהילים / Ψαλμοί both in Hebrew and in Greek? Would the differences they surely would have noticed have been ugly? Or is what gets lost and found in translation both good and bad?