Can someone please help?

1)What is the lag between current NAV and the NAV at which our BUY order gets executed? Suppose I see the market falling and wish to buy at *current* NAV, can I do it? NAVs fluctuate through out the day or it is calculated only when the market closes? Would appreciate if someone gives a link/explains the way MF orders get executed, say via a broker like ICICIdirect.

2)Can I cancel a SIP at any time? Suppose I created a SIP for 12 months and after 4-5 months I don't want to continue. Can I cancel it? Any charges apply?If it is allowed and there are no charges then why the duration for SIP at the time of its creation is not "flexible" and minimum (I saw in case of HDFC/Sundaram) was 12 months?

Thanks in advance
Deep Blue