At least I've actually done a few A3's. I know from your comments on other similarly optimistic threads that you've never even had the door card off of an 8L A3.
The speaker depth is not the issue, but suggesting building baffles and other reinforcement is well meant but impossible on an standard A3 door card, there just isn't any room, it's as simple as that.
How do you suggest the doorcard be fixed to the door? as at the moment it's just loosely hooked to the door with the only strong fix behind the grab handle.
The machined MDF fit's behind the speaker? Where exactly? Somewhere in the 2mm of available depth you have left once you've fitted the components and the required MDF spacer rings? I don't think so.
As always you are giving advice based on theory and what should be doable. My advice is based on experience and practicality. Obviously most things are possible if you want to throw huge amounts of money at it, but why bother when for £300 you can achieve a much better result for 4 hours work?