Re: select (17, 42)::s.t2 into... fails with "invalid input syntax"

If you write s.x there it will also work.

Your first and third assignments are identical in syntax/nature.  These are both the first examples here[1]

Yes, the behavior of INTO in the second assignment is somewhat non-intuitive; but covered here[2].  Probably it could use more examples.

The final form fits into a procedural flow better than the SQL-based one.  Since plpgsql allows for procedural flow this makes sense.  The composite variable reference is simply: main_type_name.field_name  Hence the second example here[1]

[1] https://www.postgresql.org/docs/current/plpgsql-statements.html#PLPGSQL-STATEMENTS-ASSIGNMENT

[2] https://www.postgresql.org/docs/current/plpgsql-statements.html#PLPGSQL-STATEMENTS-SQL-ONEROW

David J.