virtual or physical VPN-Gateway.

Don’t think so. Probably needs TOR_TRANSPROXY=1. Haven’t tried.

Likely not a good idea to use TBB because using TB over clearnet will make you close to pseudonymous. Therefore, TB fingerprint protection won’t help. In this situation, Firefox + addons is better than TB because even though you will get a unique fingerprint, you can alter (somewhat randomly) your fingerprint intentionally. TB will always have a consistent fingerprint.