Why does it say that Hashem's word is straight?
Malbim: This corresponds to natural conduct. "Devar Hashem" is what created existence - "bi'Dvar Hashem Shamayim Na'asu" (verse 6). It is based on straight laws that do not change. There is no addition to His word, or detraction from it.
Why does it say that all His deeds are "be'Emunah"?
Radak: Even if a mishap occurs, Tzadikim and Yesharim praise Hashem and affirm His righteousness and straightness. They know that it is for their benefit, and what He does is with Emunah. They thank for the bad with Simchah and a full heart, just like for the good (Brachos 54a). Bad is good for them! "Odecha Hashem Ki Anafta Bi" (Yeshayah 12:1), "Chesed u'Mishpat Ashirah" (below, 101:1).
Malbim: In Tanach, natural conduct is called Emunah. The laws are faithful and eternal. Hashem 'obligated' Himself to always conduct based on these laws, and not nullify them (permanently).
