You can get to the database manager from:
http://yourdomain.com:8069/web/database/manager
I believe your /?db=... solution should work for urls.
As for multi-domains I am not sure. I actually asked this question early February and haven't found an answer yet. https://accounts.openerp.com/forum/Help-1/question/43761
edit: My question was about multiple shops running off the same database. For your request of multiple databases you may want to run two or more separate openerp server instances configured with different ports and then route traffic to those ports for each specified domain.
edit2: Found this: https://accounts.openerp.com/forum/Help-1/question/6583/
Looks like you can specify the --db-filter parameter to select a database based on the requested domain or subdomain name.
May be this can help http://help.openerp.com/question/44521/webbuilder-site-access/