The cheaper European brands like Skoda and SEAT also likely have significant volume. Skoda, for example, sold over a million vehicles—a non-trivial 10% of VW group's total.

Audi sold something like 1.8M, probably with better margins than Skoda, so it's certainly a more important brand, but SEAT and Skoda aren't trivial either.

Just in terms of car sales, SEAT + Skoda ≈ Audi.

Not any more, Scion was shut down as a failed experiment last year.

Skoda as well - (a million cars a year)