بسم الله الرحمن الرحيم
(1) The nikah which Muhammad Rizwan did with the divorced woman while her iddah was not over then that nikah was not valid. It is mentioned in Shami:
وأما نكاح منكوحة الغير أو معتدته فلم يقل احد بجوازه .
The ruling which the Mufti sahib or Hafiz sahib of Bihar told is not correct. It is necessary to do nikah again after the iddah is over.
(2) It is not necessary for Rizwan to take permission of the first wife for the second marriage.
(3) According to Shariah when nikah was not valid with the second wife then there is no question of talaq.
(4) Talaq takes place if given verbally by force. However, only written talaq does not take place forcefully.
(5) There are too many rights, specify the rights which you want to know.
(6) Yes, it is a sin.
(7) In case of having two wives quarrel is inevitable. It should be dealt with beautiful wisdom.
Allah knows Best!
Darul Uloom Deoband