Even though I mostly seem to be talking to myself, maybe this will lead to something. Decades of psychotherapy can't be wrong, huh?
Now, another reinstall later, this time I took ebox 1.0 from the PPA. Installed ebox and ebox-network (and the dependencies). Now when I pretend to want to enable the network module, ebox informs me what it will do, see attached screenshot.
But /etc/network/interfaces is never changed, it looks exactly the same pre- and post-chrash. So is that the problem? Maybe ebox doesn't have the necessary rights?
The Dashboard gives me a list of network devices: bond0, shaper0, dummy0, eql, eth0, gre0, ip6tnl0.