I think you misinterpreted what I was saying or I didn't explain it well enough. Tor utilizing 100% CPU usage is only normal if you are pushing a LOT of bits. In this case, you probably have a system misconfiguration somewhere (nothing to do with Tor's configuration, torrc).
>"Nor, the adresses of the inbound traffic were from different adresses."
Yes, that's expected. You're getting connections from the Tor network.
>"I thought that it was not possible to force traffic through a specific predefined route in Tor"
It isn't possible. I believe I said so, or implied it. The only way to do this would be through an attack on the Tor network in general.
>"Is it possible to flood the tor port directly with for example syn floods?"
Through the Tor network, no, that's impossible. TCP relies on a 3-way-handshake which means that every connection between relays will have to be complete; therefore, in order to connect to your relay, a complete connection will have to be made. I hope this makes sense, if not, I can elaborate a bit more.
However, if someone has a hold of your IP, they can run a portscanner and then determine your relay port (which is on the internet for all to see.) Therefore, you can be attacked, but not through the Tor network.
>"If yes; is there an iptables rule which will reduce the amount of connection kept in the syn state?"
First of all, no. And second, that's not how you deal with a SYN flood. If that rule was implemented, it would just be easier to take your port offline.