RSPi and dedicated expansion board to read real time data?
You can't have real-time on Linux systems. You should use a "bare metal" microcontroller instead if you need real-time.
No idea. Lots of questionable hobbyist projects out there.
You need to know the bus "infrastructure". Which nodes are there, what CAN bus protocol are they using, what's the baudrate, 29 or 11 bit id, where is termination placed, what physical connectors to use, stub considerations if you plug in on the bus, where to supply the board from etc etc. And obviously, connect signal grounds (you'd be surprised how many quacks there are who don't).
Aimlessly sending CAN data on a live bus designed by someone else, when you don't know neither the protocol nor the real-time considerations made by the one who designed the bus. Doing so is plain dangerous.

Examine if the ODB port got supply pins along with the bus, then treat them with care. Accidentally shorting those with other pins might damage electronics if you are unlucky.

Apart from that, you can't really cause harm by just passively listening in on a CAN bus.