The reason for the roof is simple, the engine puts out a lot of torque and with the sunroof it looses a lot of body rigidity. The reason your mini can do it is the length of the car and the type of cage they have designed for it. I would avoid the sunroof MS3, if it was me. It may be ok now...
When dealing with an issue like this you might want to consider calling Mazda Canada directly. If it is true you did all the maintenance on time, you should register your complaint with them, and provide them with proof. You will probably not get very far, but note who you spoke to and what...