Sadly I would have to switch to one of my ISPs business offers. That would cost more than the double and deliver some telephony service I'd have to pay but would not use. Anyway, as I found no clean way doing this with pfSense, I tried to verify that setup with OpenBSD and I'll stick with that for now. Using ifstated together with CARP you can simply ifconfig down the external interface automatically so it will not interfere with anything on the outside.
It's a pity I'm not really good at anything related to programming but a shell little scripting, otherwise I would try to implement some system triggers (i.E. Disable Interface X) in case of CARP failover for pfSense. I really like pfSense, it's the best Open Source Firewall distribution there is. Maybe someone else likes the idea and does implement it