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?