1)
Why do they say "Lo Yitachen Derech Hashem"?
1.
Malbim: People are the opposite ? they remember the early deeds, and judge according to them.
2)
What is the meaning of "v'Hemah Darkam Lo Yitachen"?
1.
Malbim: [People are skeptical about Hashem's way because they judge based on the early deeds. Just the contrary,] their way cannot be [proper]! Since he repented from his early deeds, one must pass over them and tolerate his sin!