السلام عليكم
Yes I agree 100%, having possessed knowledge/faith/belief and then committing wrongdoing under no compulsion, is seen as an act of malice. It is the understanding and intention which is crucial. For example, if one sins but is ignorant of the fact that it was wrong, he cannot be held accountable for that action and nor will Allah hold him to account. Yet if for example, one was to fully acknowledge the premise of One God and then strayed into polytheism, is this act worthy of forgiveness? Yet, Allah is still الرّحمان...
إِيمَانِهِمْ seems to be emphatic in this case.
I also agree with your translation, and then after thinking about the possessive construction, the verse seemed to give new meaning...
e.g. كَيْفَ يَهْدِي اللَّهُ قَوْمًا كَفَرُوا بَعْدَ إِيمَانِهِمْ
Translation: 'How Allah (still) guides people (who) sinned after (possession of) their faith.'
This verse then could be highlighting the Mercy of Allah, even after one has knowingly commited wrong. e.g. drinking, lying, fornication, stealing, etc. And has repented therefter...
However, I believe your translation is stronger, as gaining forgiveness in this case is far more difficult.
مع السلامة