1) Is it allowed to have anal sex with a wife during her periods with her consent ?

2) If the wife is in her periods and had anal sex, does she has to do Ghusl of Janabat during her Periods,
i.e. will she do Ghusl of Janabat immediately? While in her periods?

3. If the above answer is (2) NO, then pls advice will she stay in the state of Janabat and how will she come out of it?

It’s not allowed to have anal sex, it’s Haram and if anyone has done during his wife’s menstrual period then when she will become Pak from her menses, she has to do both Niyyat (Ghuls of Hayz and Janabat) at the time of One Ghusl.