Required margin by NSE : SPAN + EXPO
Minimum required after which they charge penalty is : SPAN
1. Even for overnight, if you are making profits you don't have to worry about the incremental increase in margins.
In your case, with 1lk as margin you making 5lk in futures as MTM is not a realistic example. If with 1lk you have bought 4lots of nifty in NRML, you will need 2500 points movement in Nifty to make 5lk as MTM. But yes in such an extreme case there could be an exchange penalty, but we won't square any positions off if you are making profits.
Why penalty?
You have 1lk and 4 lots. Exchange penalty starts if you have less than 18k/lot(SPAN). If market went up 2500 points, The SPAN might go up to 27k/lot. You have 1lk where as SPAN required now will 27 x 4 = 1.08lk and you are short Rs8000. So exchange would put a penalty on this short amount, I guess b/w 0.5 to 1%, so you will probably have a penalty of Rs 80, guess you wouldn't mind if MTM is 5lks.
2. If you are making a loss, then it is at the discretion of our RMS team, especially if you are utilizing the funds completely. So if 25k is what is required to hold overnight, if you are loosing more than 15 to 20%, our RMS team could square your positions off. Again as I said, at the discretion of our RMS team.
3. You make profits, no one is going to square off your position early. But yes if you are making losses because the market volatility is extremely high, we might be stricter in terms of margin requirement.