Ad 1:

Yes, certainly. Just define the foreign table as

CREATE FOREIGN TABLE view_1_r (...) SERVER ...
   OPTIONS (table 'VIEW_1', schema 'USERB');

Ad 2:

Yes, certainly. Just define a foreign table on the Oracle table and insert into it. Note that bulk inserts work, but won’t perform well, since there will be a round trip between PostgreSQL and Oracle for each row inserted.

Both questions indicate a general confusion between a) the Oracle user that you use to establish the connection and b) the schema of the table or view that you want to access. These things are independent: The schema is determined by the schema option of the foreign table definition, which the user that connects is determined by the user mapping.