No, Air China has nothing at T2. They're operating out of T3.

Yes, they allow online checkin

http://www.airchina.com.cn/en/servic...c_cities.shtml

Online check-in for Air China-operated flights departing from Beijing for other parts of Mainland China and Asian cities (except Thailand, United Arab Emirates and Myanmar) is available from 20:00 two days before flight departure to 1 hour before flight departure.

You'll be fine. Do online checkin while still at home. Once landed in T2, show the eticket and/or your boarding pass, get cleared for TWOV, go to the shuttle bus, ride over to T3..

Nothing to worry about.. your chance to make the connection with CA is above 95%. And you'll know (almost) for sure if you're making it or not while still in BRU. If you're departing - more or less on time - then something MASSIVE would have to happen for you to miss the connection.