AFAIK, a missing middle name is always OK. Just too many ways for this to get screwed up across systems that have different concepts of names.

What I do not know is how the multiple surnames works. Take a look at the line near the bottom of the passport that has your name in machine readable form, If the ticket matches the first 2 names listed there I would think you are fine. if not, I would be a bit concerned.

You might call them and see what they can do. If it is obvious the name is really the same, I would hope they can override the blanket restriction on changes (or perhaps a free cancel and reissue if the system does not allow the change at all).