How can I findout detailed information about the DHCP release of the WAN adapter (time to next renewing and so on).
If you’re using Winbox you can locate it under IP → DHCP Client → DHCP Client tab → double click at desired interface → Status tab.
To see details from console use /ip dhcp-client print value-list.
Thanks.
Is it necessary to divulge the hostname and client-id to the wan dhcp server?
The client-id is any MAC-address, this I think is necessary. 
Is there any RFC about what is really necessary?
If I do not want that the dhcp server sets my local dns entries, can I control this behave by setting any dhcp-options?
You also use opendns and google dns servers on the screenshot - does a locally set dns entry override the dns entries given by the dhcp server?