No its not, the Alfa head unit accepts a signal from the Bose system to reduce the speaker output so that the Bose can amplify it instead.
yes, i believe the bose amp relies on a 'low level' (low voltage) audio signal as its input. 'how low' i don't know (ie: the equivalent of a '2V' preout, or 4V, or much lower?).
it means that you may need to modify the connection between the new headunit and the bose amp. for example, you could use the RCA preouts from the new headunit, add an RCA cable, then splice this into the factory cable going to the bose inputs. this would mean connecting the Front Left RCA + and - wires to the FL +/- wires going to the amp (ditto for the other channels). this obviously requires you to know what wires in the factory cable are for which channels, and for + and -.
if the signal voltage from the new headunit is lower than the OE blaupunkt, it means the volume won't go as high as the factory system. if the voltage is higher, then you risk pushing the amp into 'clipping', where the input signal is too high, and the amp distorts the output signal as a result. this poses the risk of overpowering the factory speakers/sub and frying them. though this usually manifests as gross distortion that is not hard to hear.
ultimately, there is not guarantee it will be a simple plug-and-play change-over. more likely the contrary, and either require you to have knowledge of how to modify the connection, or pay a pro installer to do the job.