Salaah

Filling the gap in the saff if the muqtadi leaves during salaah

Q: There is a rule in Bahishti Zewar that it is makrooh to stand alone in a row. The person should pull a person back from the row before him. What happens to the gap formed by the exiting of the said muqtadi to fill the previous row?

If a Muqtadis wudhu breaks during prayer, should he just leave and leave a gap in the saff? What should he do in this regard?