1)
Why does it say "Lo Achalel Brisi"?
1.
Radak: Even though he profanes My Chukim and does not guard My Mitzvos (I will not annul My Bris).
2.
Malbim: This corresponds to "Karati Bris li'Vchiri" (verse 4), which was due to Chesed.
2)
Why does it say "u'Motza Sefasai Lo Ashaneh"?
1.
Malbim: This corresponds to "Nishbati l'David Avdi" (verse 4). Even though [I swore] because he was Avdi, and now [his sons] departed from My Mitzvos, I will not deviate, after the promise left My lips.
