It is possible to buy PE on Starlux (just did that), for the same price as on their own website. However, one time the AS website search only showed Y and J.

As it comes to other carriers, it doesn't even seem possible to book e.g. BA SEA-LHR-AMS return. I'm wondering if you'll get lower fares and more route availability if you call in to book.

Agreed that right now booking on AS website isn't really useful for most cases. Hopefully this will improve.