The Common ID is used only for paging coordination: when the MS is in a packet transfer, it is still able to receive CS Pagings.
Your questions, my answers:
1. Does BSC send this IMSI over Air Interface to MS.
— No !!!
2. Is there any possibility to hack the IMSI by some one.
— Yes, a hacker can always hack IMSIs from a network, but the usage of the “common ID” won’t make it easier. IMSI are not transmitted on the Air interface, except on very specific cases (AFAIK : roaming MS or new SIM cards, at first attach only)
To secure your network as much as possible, use A5/1 or better A5/3 encryption, TMSI Reallocation, TMSI paging, … (i’m no expert on the topic)