I have a similar configuration like your #1 with no problem. I configured that under
Configuration & Details -> Routing -> Sender-dependent transports
There I configured my relay server and under Configuration -> Mail Setup -> mydomain, I used that entry as a Sender-dependent transport
( See Documentation 
here ) mailcow.github.io
Concerning your #2 Problem, I think that should be able to solve that with global Transport Maps. (Configurable right under Sender-dependent transports)
Hello,
I am using Mailcow for a couple of time now without a problem. However, while looking through the Postfix logs, it came to my attention, that mailcow is trying to send a message from watchdog@mail.mydomain.com which gets spam filtered:
NOQUEUE: reject: RCPT from [77.22.xxx.xxx]:35122: 550 5.7.1 Service unavailable; client [77.22.xxx.xxx] blocked using zen.spamhaus.org; from=watchdog@mail.mydomain.com, to=User@mydomain.com, proto=ESMTP, helo=<mail.mydomain.com>
The mailcow server is running under mail.mydomain.com and is handling mails for @mydomain.com. Even though, the server itself is running from a dynamic IP, I use a relay server from my domain provider to circumvent any trouble with spam filters who block incoming mails from those addresses. Maylcow itself however tries the direct approach and ignores the transport map, thus getting trapped by its own spam filter. I tried to add the domain an address in the spam filter whitelis, but to no avail.
Maybe someone can help here?