Why does it say "Lachen Ish ki'Drachav Eshpot"?
Malbim: Because you change your ways, I will judge you, a man according to his ways. I will arrange his current path opposite his previous path. This increases the reward of a Ba'al Teshuvah, according to the strength of the Yetzer ha'Ra that he was used to; the reward is based on the pain. It also increases the punishment of a Tzadik who abandons the good path that he was used to.
Why does it say "Shuvu v'Hashivu"?
Rashi: You should return, and cause others to return. Sha'arei Teshuvah 1:50 - one of the principles of Teshuvah is to induce others to repent, according to his ability.
Radak citing Targum Yonasan: Return to serve Me, and I will remove from you service of idolatry.
Malbim: Strive to do Teshuvah amidst love. Then, you also 'return' your prior evil deeds to be merits.
What stumbling block of sin will not be?
Radak: Refraining from repenting.
Malbim: If you repent amidst fear, your prior were not atoned, just they became like Shogeg; it is as if you stumbled in a sin b'Shogeg. Repent amidst love, and no stumbling block remains!
Idolatry. (So it seems that Targum Yonasan explains; refer to 18:30:2:2. - PF)
