I used the original speakers as adapters for some new 13cm JBL 2 ways for the doors in my daughters car.
I just used a set of side cutters and cut out the centres of the old speakers after using a sharp knife to seperate the cones from the foam surrounds before screwing the new speakers to them - saved £13, as the OEM speakers wouldn't have gone back in. I expect to do the same when she upgrades the rear speakers.
I would expect them to be fixed in the same way as the doors - 3 self tappers
I've seen glued construction, but not glued in. If it went wrong under guarantee, they wouldn't want to replace the whole module, and it would be difficult to manufacture on a production line due to the time factor of the glue setting (unless it was hotweld)
The door speakers are made up with the adapter as an integral part of the speaker. The cone is glued to the 'adapter'. I had to use a scalpel and cut through the old paper cone (not the foam surround) before using side cutters to separate the magnet/voice coil from the adapter surround. I then screwed the new speaker to this.
The reason why I did this was because Halfords had a problem sourceing the adapters, and when I went to a local car hifi specialist, this is what they said they do with corsa's when replacing the speakers. They don't bother with the adapters, and because the original speaker surround is being used, it is guaranteed to fit and give a reasonable seal.
I'd not use 6"x9" speakers either if you were to consider it TBH as unless they have a custom enclosure built underneath them, they will interfere with a sub enclosure unless it was ported to the passenger compartment (and they stick out like a sore thumb unless you had a custom shelf made up also.