A bit of a "closing" note here: today, seeing fare availability remain the same, I insisted via Facebook Messenger. After a couple of back and forth Z "became available". I've processed the change and paid 200 EUR. Not a bad outcome given this ordeal started at 3055 EUR. HUCA (with the C for chat in this case) was helpful in this scenario. Sad, however, I had to do it nearly 10 times.