Is it permissible to adopt the fatwa of Ibn Taymiyyah regarding the non-obligation of making up prayers and fasting that were missed due to masturbation, out of ignorance of its ruling, and is this considered seeking out concessions? And is it permissible to make up only the prayers and not the fasting?

Masturbation is forbidden and necessitates ghusl (ritual bath). It invalidates the fast if semen is discharged during the day. However, the fast is valid if it occurred at night and you did not perform ghusl until morning. There is no sin upon you for your ignorance of its ruling. It is more prudent to make up the prayers, although Ibn Taymiyyah held a strong opinion against the necessity of making them up. It is permissible to follow Ibn Taymiyyah's fatwa because he is among the trustworthy scholars, especially if you are convinced by his fatwa and if there is a need. There is no disbelief in abandoning prayer due to ignorance, and the fast remains valid and is not nullified by committing an act that breaks the fast out of ignorance of its prohibition or of its being an invalidating act.