Is it possible to make one core-node accessible under several ip-addresses? (with the result that other nodes can not recognize that it is the same node under two different ip-addresses) If this is not possible: Why?
Yes, it is possible.
You simply need to listen on the different IP addresses. What is necessary to do that and the steps required depend on your network configuration and hardware capabilities.
The trick is then to get your node to advertise all of the different IP's, which on some node software I think (from practical experience but, without going back actually to confirm) that you can just specify multiple external IP's.
I am not sure that this accomplishes much, however. If you have the bandwidth you can just modify the node to allow a high number of incoming connections and use a single IP.
Also, I suspect, others may confirm, that the default connection rules have subnet spacing integrated so that the same node will not connect outgoing to all connections in the one subnet.
If you control traffic and DNS on your network it is trivial to ruin Bitcoin via a number of methods as you could the operation of any application.
you are talking tcp/ip network stack and name resolution here. It can be done, you can change your IP as often as you want on your system. For a core node this doesn't make sense, so this looks like someone wants to find a way to destabilize the network. Or where you tricked by someone? On DNS and tcp/ip stack, this is the wrong area here - this is not a network stackexchange...