The grill is pretty close to the face of the door panel and the thickness of the door panel doesn't seem like enough where you could push over 1/2" into it without hitting the back of the grill. That is your biggest issue here along with a few smaller ones. I think the back clearance will be okay based on the measurement you show.
The smaller issues are, how do you secure the speaker to the adapter. Since you've cutoff all the mounting tabs, you're basically stuck with using some kind of adhesive or glue type product but then you've gone through all this effort and the weak link is the adhesive or glue which puts you in the same situation as Technic's dynamat solution. So you've gone through all this work to end up in the same place you would have been with $5 worth of Dynamat. I think if you're going to expend that much effort, maybe a solution to mount the speaker from the front side of the door panel would be something worth looking at. It would probably require that you install a non-OEM grill and possibly drill a couple of holes into the door panel but at least you have a secure mounting system to work with.
I'm assuming being able to put back the OEM speaker is not a concern since you're already hacking up the OEM speaker. Depending on where the new holes end up, it might be exposed if you try to reinstall the factory grill. Unfortunately, I don't see any other way of doing this without doing any mods to the door panel unless you go with the dynamat method.
2016 Porsche Cayman GTS - Sapphire Blue / Black Full Leather Interior / Carrera S Wheels