Will the insistence on repentance, seeking forgiveness, and following up a bad deed with a good one intercede for someone who committed the sin of masturbation, despite their inability to stop it, and despite the fear for them from this sin even with their religiosity?

There is a great difference between adultery and masturbation. Adultery is a major sin, whereas no scholar has considered masturbation a major sin, and the harms resulting from each differ. However, persistence in a sin, even if it is minor, elevates it to the level of a major sin. Your relative's lack of persistence in disobedience and his seeking forgiveness, coupled with his performance of good deeds, will benefit him and expiate his sins, God willing.