When I worked at Vauxhall for 7 years I cant say I have heard of any Zafira's in that time going on fire. From what I can gather the cars that have gone on fire are at the older end of the Zafira B production so there is a good chance that poor parts are used or pollen filters are not being replaced or even leaves are stuck in the motor assembly.
The heater resistors are currently on back order and the recall status is pending.

I recall some Corsa D's having a problem with the wires goi g to the heater resistor melting but I don't recall this happening on the Zafira B