Is masturbation absolutely impermissible, even in cases of extreme necessity, as Sheikh Muqbil al-Wadi'i stated, or is Imam al-Shawkani's view the correct one?

Necessity permits the forbidden to the extent that it repels it, such as the retention of semen that causes illness if no permissible alternative is available. Fasting is a permissible alternative for those who cannot marry, as the Prophet, peace and blessings be upon him, guided. Some scholars have absolutely prohibited masturbation, while others have permitted it only when there is a fear of severe hardship (such as fornication or sodomy), and patience is better in all circumstances.