A woman used contraceptive pills, and her menses exceeded (15) days, is it permissible for her to fast?

The maximum of menstrual blood is (15) days, but if it exceeded that due to a medication, or the like, then it is Istihadah (bleeding outside the monthly period), and in this case she should perform Ghusl (ritual bath), pray, and fast. Moreover, she is obliged to make up for the missed fasting days which exceeded her regular menses. And Allah Knows Best.

Is it permissible to make up for the missed fasts of the deceased?

A deceased`s missed fasts should be made up for by his/her guardian. It is also permissible to make up for the missed fasts of a deceased relative, and to pay a ransom in expiation for the latter`s missed fasts, which is feeding a needy person for every missed day. However, the guardian`s permission need to be sought by the non-relatives of the dead to fast on his behalf. And Allah Knows Best.

Is it permissible for the woman to use her mobile phone while on the street?

All Perfect Praise is due to Allah, The Lord of The Worlds 

Modesty should be observed by the woman at all times, so she is free to use her phone while on the street as long as it is done within the limits of Islamic manners. And Allah The Almigty Knows Best.

Is it permissible for a sick person to pray while sitting on a chair?

All perfect praise be to Allah, The Lord of the Worlds                                                                                                                                                                        It is permissible for a sick person, who is unable to stand up, to offer prayer while sitting on a chair. If he/she was unable to prostrate while on the chair, the he/she should pray while sitting on the ground, but if he/she was neither able to bow (Roku),nor to prostrate, then he can do them while sitting on the chair. And Allah Knows Best.