Is it obligatory to repeat the prayers after discovering dried semen on the body after performing ghusl, and does dried semen prevent water from reaching the body during wudu and ghusl?

Masturbation is forbidden. If there is a small amount of dry semen clinging to the hand, the matter might be mere waswasa (whispers from Satan). Assuming its presence with certainty, this is a minor barrier that does not harm, according to some scholars like Ibn Taymiyyah. It is permissible to adopt this view here because the matter has passed and cannot be remedied. The order between the current and the missed prayers is recommended by the Shafi'is, so it suffices to repeat only the invalid prayer. It appears from the question that there is waswasa, and it is advised to persist in supplication, disregard the wasawis, and consult a psychiatrist.