My questions are the following:
a) Does it damage the security of my Armory-wallet by exporting some Private Keys? (In other words: Is it possible in any way that someone could calculate my seed with some of my private keys?) (Ofc. I am aware, that i can not use this single address anymore!)
Yes, it totally compromises your wallet! Just one private key together with the watch-only wallet, or two private keys together are enough to calculate all remaining keys in your wallet!
Are you sure about that? I would have not expected this, so glad I asked bevor acting.
As it would be very difficult for me to create a new Wallet (because of the high security steps I took in order to properly have backups in different places) i guess i will install a bch-node and move my funds via armory. Still have to update my coldwallet then, but it might be the easier way.
Or is there any way i can run my online-armory against a bch-node on the internet? So that i dont need to sync a new node all day long.... I know, it has some loss in privacy included, but i'll take that.
EDIT: Btw: Where can I find the latest Raspberry-Offline version, including the bch-signer?