Was what I did—praying individually while following the Imam, then leaving the place immediately after finishing the prayer to avoid fitna—correct, given the women's refusal to straighten the rows and close the gaps?

Straightening the rows in prayer is obligatory for both men and women alike; it is part of the perfection of prayer and includes joining the rows and not leaving gaps. The prayer of an individual praying alone behind a row is invalid without an excuse, whether male or female, due to the generality of the legal evidences. However, if the isolation is due to an excuse, such as there being no space in the row or praying behind men's rows, then the prayer is valid.