1) Yes, RJ11 is the small brother to the RJ45 so an adapter would ensure the correct wires are connected.
2) We would all need to know the exact network configuration in your new office block, to know if it is even possible for you to daisy-chain off the switch-board, in the manner you describe, let alone advice how to install and set things up.
You _must_ speak with those responsible for the office block's network.
3) What are you trying to plug into a BT phone socket?
4) You scare me.