Jamaat Salaah and Imaamat

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 ?

Pulling a person back from the front saff in Jamaat Salaah

Q: If a person comes last in the masjid and there is no space in the first row

1. Should he slightly pull a person from the first row to stand next to him?

2. If the person from the first row comes back to join the person standing alone in the back, what should be the duty of the men who were standing left and right of the person who went back? Should they remain in their places or move closer? What is the proper way to fill that space?