The Mazda 6 of all generations is utterly unrelated to the Mundano, with the exception of the Gen 2 which used a Mazda redesigned variant of a floor pan which Ford themselves used in their own redesigned form. Mazdas own in house (and quite troublesome) engines, different structure, completely different suspension arrangement. I doubt there's so much as a sump plug washer common between the two.
The X-Type was Mondieu relative. The Jaguar engineers took a Mondeo, probably the best car in its.class at the time, and made it worse in every way, then stuck a stupid XJ6 front end on it.
As for fuel fillers... I don't have this problem with veg