1)
Why do they say that Hashem's ways cannot be?
1.
Malbim: How can Hashem turn sins into merits? How can a Rasha become a Tzadik? How can a Tzadik's sin (after he began to go on an evil path) be worse than a Rasha's, and the Rasha's merit more than the Tzadik's?
2)
Why does it say "Darcheichem (plural) Lo Yitachen" (singular)?
1.
Radak: Each of your ways [cannot be proper].
2.
Malbim: Your ways do not have one content; you change them constantly.