Is the disagreement among scholars regarding the obligation for the repentant to erase the traces of his sin an issue open to ijtihad, allowing one to adopt any opinion therein, or is it a matter of dispute? And do those who advocate for the obligation of erasure mean that sins continue to accrue to the repentant as long as the traces of his sin remain?

Guidance for someone afflicted with waswasah (whispers/obsessive thoughts) is to cast them away and not pay attention to them, for Allah does not burden a soul beyond its capacity. A repentant person who used to call others to innovation (bid'ah) or misguidance must clarify to those affected by his misguidance that what he called them to was contrary to Islamic law. This is similar to the repentance of the People of the Book who concealed what Allah had revealed; they repented, reformed, and made it clear. You have previously advised those whom you called to sin, and by doing so, you have discharged your responsibility, and none of their burdens will befall you. So, abandon the waswasah, for indulging in them leads to great evil.