Open questions are: 1, 2, 7. For 2: yes, I have an old dual CPU (each single core) motherboard as my client PC (running Vista 32). But I didn't give any thread limitations and I was talking about the official client, not about any miner. So the question remains why it's using CPU at all.
What version of the client are you using. My understanding is that old clients do have miners in them, so that might be why you are seeing this. I am using 0.3.32-beta and there is no miner.
And if I understood you correctly, it's not worth running *any* mining software on the server (although the server is running anyway), because the miner would increase CPU usage causing more electricity costs not worth the results. Correct?
That's correct, but at the same time I encourage you to download and try it just as a proof of concept to understand how it works. I think doing these steps is worth it.