No subject

domain are equivalent, don't try to guess how they do their
administration, they might not even run BIND.

The server listed in the SOA is significant to clients only when
doing dynamic updates.

> - Several domains can share one Server. (One box with one DNS-Demon)

Yes.
 
> Everytime I request a .de-domain my local server can´t
> resolve it, so it does at it is told, it requests  a root-nameserver in the
> .domain for the name requested or at least the IP of server that is
> authorative for the .de domain.

The DE name server records will be cached for one day, so you
would save yourself one query a day approximately.

A large German ISP might make a useful contribution if they
acted as a secondary for the DE domain, but that is really up to
the administrators of "DE" to work out, looks to me as if the
ownership of the "DE" domain is spread thinly enough.