The operator or some member could distribute a "installer" which would perform these actions automatically. I expect most people wouldn't care if it benefits their bottom line and others could rationalize it under the guise of a defensive measure.
Also, these interference nodes wouldn't have to be big time hashers. A botnet could be put to use for the simple purpose of diluting the number of useful broadcast recipients or triangulating on DDoS targets rather than hashing directly as is more often suggested.
You are confusing mining clients with network nodes.
That's true.
I did refer to the machines producing 1/3 of the total hashrate as "1/3 of the network".
Is there any way to estimate the actual network size?