Rollover activity generally peaks during the last week of the expiry month, especially on F&O expiry day (last Thursday of the month), when traders shift positions to the next month.

Rollover in the stock market refers to carrying forward an existing position in the futures contract from the current month to the next or far month contract, just before expiry.

Yes, margin is blocked as per your position. SL reduces your risk, but initial margin is still needed.