Answer
(Fatwa: 1894/D=1491/D=1430)
(1) Calculate Zakah at the completion of one year, then pay Zakah in lump sum or bit by bit around the year, but delaying it unnecessarily is not right lest you should miss the same.
(2) If you already owe nisab, so you have to pay the Zakah of amount gained during the year with the nisab.
(3) It is not necessary to inform, rather you can give him in the name of gift.
(4) Give Zakah as per the market value of the stock at the time of completion of the year.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband