This seems to be the case. I had exactly the same issue as Tim Cowell and ended up booking on an OTA. But at least the OTA allowed me choose seats as part of the payment process.

From what I gather, MU only allows you to access website functions if you buy directly from them. It's crazy. And don;'t bother with the app unless you apeak very good Mandarin.

God knows what online checkin will be like - I presume I will need to do it at the checkin desk.