(1) If you are suffering from the problem of ihtalam (wet dream) very often then you should consult any expert Hakim. Also recite Ayatul Kursi, Surah Naas and Surah Falaq while going to bed in the night and blow over your body. Also sleep in the state of wudhu and on your right side. Likewise, read any Islamic book i.e.(Bahishti Zewar or Maariful Hadith or Nashr al-Teeb) before going to bed. In-shaAllah, your problems shall decrease.
(2) According to Shariah, ghusl becomes wajib if semen is discharged by ihtalam. Salah is not valid without ghusl. Hence in the above mentioned question if you discharge semen at the time of ihtalam then ghusl shall be fardh on you. It will not be enough for you to wash the part below the navel.
Moreover, it is advised to recite surah al-Tariq while sleeping in order to prevent excess ihtalam.
Allah knows Best!
Darul Uloom Deoband