Meaning of mannat

Q: My daughter has a condition called chronic migraines - something she has had for almost 6 years now - Alhumdullilah she has trained herself and has increased her tolerance for pain. However there are some days where the pain is intense and she is unable to cope. Can she take out a mannat during the first 10 days of Muhurram? What are the rules and conditions for taking out a mannat? Can I, as her mom take out a mannat on her behalf? 

Threatening to poison one's co-wife

Q: I married a Muslim man as a second wife. When his first wife was informed, she threatened to poison me, my (our) husband, the children and herself. I am new to Islam and my husband is teaching me about the religion. I have no idea what to do as my husband now fears for the lives of his children as well as our lives. Can she force him with these threats to divorce me? Please tell me what to do! 

Using interest to offset bank charges and disposing of interest money

Q: I have both a current account as well as a savings account at the same bank. On both the accounts, the bank levies bank charges, but the savings account is interest-bearing.

In this situation, can the interest received on the savings account be used to pay the charges levied on both accounts by the bank?

If not, then to whom should the interest be given? Can it be given to non-Muslims and even to street beggars or beggars knocking on doors every week?