Salaah

Performing qasar salaah when studying safar distance from home

Q: I take Pietermaritzburg as my home (Wathne Asli), however due to me studying and being married, I live in Isipingo Beach and take Isipingo as my Wathne Iqaamat. If I stay in Isipingo for less than 15 days will I be a musaafir in Isipingo? Doesَ my wife have to follow my intention as taking Pietermaritzburg as my Wathne Asli and Isipingo as my Wathne Iqaamat, or can she take Isipingo as her Wathne Asli?

Completing the salaah individually if the imaam performs 3 rakaats

Q: Our imaam offered 3 rakahs instead of 4 rakahs in asr farz Salah and ended the salah. No one interrupted him for his mistake and all the people ended offering 3 rakahs, then people said it was only 3 rakahs completed so the imam repeated the 4 rakahs again and every body prayed 4 rakahs again.

My question is if a person is behind a imam in the above case and is aware that the imam has made a mistake can he individually continue the same Salah and complete it ?