What is the meaning of "Al Tatzel mi'Pi Devar Emes"?
Rashi: Emes should not separate from my mouth. This is like "va'Yatzel Elokim Es Mikneh Avichen" (Bereishis 31:9).
Radak: Do not separate Emes from my mouth, rather, accustom my mouth to speak Emes. If You will not, it is as if You separated me from Emes, for You can help me and open my mouth for the good - "Hashem Sefasai Tiftach" (51:17).
Malbim: I should not deny my sins, rather, I should admit to the Emes.
Why does it say "Devar Emes Ad Me'od"?
Rashi: In all my rulings and judgments, I will not stumble in one of them.
Malbim: "Ad Me'od" refers to "Batachti bi'Dvarecha" (42) - therefore, do not distance Emes from my mouth. I will answer my revilers that I trusted greatly in Hashem's word and promise. "Al Tatzel mi'Pi Devar Emes" is a Ma'amar Musgar (parenthetical comment; refer to 119:43:1:3).
What is the meaning of "[l'Mishpatecha] Yichalti"?
Rashi: I hoped [Radak - that You will teach Your Mishpatim to me].
Malbim: I hoped that You will do Mishpat. This that Avshalom pursued me, based on Mishpat it atones for sin, and You will return to have mercy on me - "Asher b'Ha'avoso v'Hochachtiv b'Shevet Anashim" (Shmuel II, 7:14); "v'Chasdi Lo Afir 1 me'Imo v'Lo Ashaker be'Emunasi" (above, 89:34).
Surely this is the correct text, like Malbim writes below (75). Many editions have a printing mistake due to similarity to Shmuel II, 7:15. (PF)