Yes, the permission of his wife is required for it to be permissible for him to marry her niece. The man does not need the wife's consent to marry her aunt. There is no difference in the two scenarios between the two types of marriages.
Masturbation is prohibited, and whoever commits such a sin should repent and seek forgiveness from Almighty Allah, but it is not obligatory on him to perform the ghusl with cold water.
When it is possible to pay the khums to whoever deserves it or to the agent, one should hasten to do so. One can use the rest of the money whether he paid the khums or not.
You do not have to do anything, and if it is given to you, you can take it.