I'm forwarding mail from another domain to one mailbox on my
citadel server.. I'd like to be able to reply to the emails with
the forwarded domain name instead of the citadel server domain..
is this possible?

Not likely to work with Citadel or any other mail system, unless the other domain has your system listed in its own SPF and DKIM records. Otherwise your outbound email will be assumed to be a forgery by the receiving mail systems.

I checked the documentation on how to change the log level (e.g.
to x5) but I do not know how to configure it with my debian
system.  It must be a very simple citserver command.  Any help
appreciated.

Citadel Server sends out logs at "every" log level. You have to configure your syslog program (syslogd, rsyslogd, systemd-journal, etc) to do something with the higher level logs.