PostgreSQL FDW: is it necessary to specify sever and user for every link to a database on the same server (with identical user privileges)?
Yes.  You can't use a foreign server without creating it.  And you would have to create one for each database you want to link to, as dbname can only be specified at the foreign server level, not at the level of individual foreign tables.  And each server needs its own user mapping, they can't be shared.