After starting this topic and complaining about this frustrating situation, I must be fair and let you all know that I have been helped out by the M&M hotline in Germany.
The Belgian hotline was not really helpful, but some very nice lady from Germany helped me out and managed to book me and my family a business class flight to Mexico City.
She understood my disappointment, asked if it was ok if she calls me back later.
A day later received a phonecall, she told me that she found something and if it was ok for her to book it.

So yay, we got helped out. Very grateful that this lady was really really kind and got us on this flight.
But still my feeling about these so called bargains didn't change, if you show off with promotion, it would be a good idea to actually promote something to make your loyal customers happy and not to get them frustrated...

Happy I got helped out, but at some point, I will be in the same situation again as I was earlier.