my flight arrives Shenzhen Airport at 12:15pm

I am planning to take MRT via Line 11 from Airport to North MRT station and change high speed rail to Xiamen. How long does the MRT journey takes and what is the time I require to book my HSR ticket? Many people telling me that the HSR tickets sold out very fast (I can see some train number already sold out and it worries me that I may not get tickets for 6 pax when we arrived at HSR station that day. Also, I heard that HSR prohibit the passengers checked in liquid in their luggage. We certainly will bring liquid, can we have our luggage checked in as cargo like flight? Thanks.