What should someone do who has indulged excessively in sins and transgressions, then returned to following his whims, and believes that Allah has sealed his heart, and wishes to repent, but is weak in the face of masturbation and pornography? Is it permissible for him to abandon his home and parents to distance himself from an environment of sin and achieve sincere repentance, knowing that his parents might be affected by this abandonment? And would he be sinful if any harm befell them?

It is not permissible to abandon a needy parent and expose them to danger unless there is a sibling who can care for them. The door to repentance is always open, and Allah accepts it no matter how many times the sin is repeated. And whoever strives against his self, Allah will grant him success in attaining His pleasure. Your تصور (perception/imagination) that your presence at home prevents you from committing (to good deeds) is a delusion. The advice is to take the initiative with sincere repentance.