I've completely revamped this post since I found the answers to my questions.

I was following this excellent guide, but I didn't execute the last steps (export DISPLAY=:0, disable crossfire and restart X) because I thought it didn't apply to my setup. Apparently, it did apply.

Furthermore, adding the -v option to the command line allowed me to run the miner just fine :-).

2.1.

Another peculiarity, I've installed a 32bit version of Linux (I don't have any optic drive, so I set up the HDD of the miner from a VM on my x86 Mac).