1) You are right. You can make several experiments, for instance starting form a single peer with a single operating system, starting with a certain percentage of operating systems, starting with a peer for each operating system. It can be the occasion to perform more experiments.
2) Again, you are right, the term removed is not defined in the text. And yes, it means patched, that is the node will no more spread the virus.
Thank you for your clarification requests
Laura Ricci