For me the false floor was just to hide all of the wiring (of which there is a lot) as I wanted the amps to have as much air around them as possible and like you I needed something decent to screw down into rather than through the boot floor. I quite like to see everything laid out, but for you I can see why it was a problem.
The carpet I used was a spare boot carpet for the car that I had in the loft, so I cut it out to fit and then used some spray adhesive (I too was surprised with how strong it was).
The quick released connector for the speakers is just a plastic connector with male/female banana plugs within it. (link below)
Unfortunatly I cant get a double din unit in there (not quite big enough). I did have a single din pioneer in my other car but got rid of it after two months as I was so unhappy with the sound quality, so I'm a bit put of by them. ideally I would like another Sony. Perhaps this one.....http://www.halfords....-dab-radio#tab3 I really need one with three rca outputs (front, rear and sub) and this one offers that.
I was the same as you at first, just get a sub in there, but then found myself thinking "If I do this next bit then it will be so much better"
My boot space is basically the back seats I rarely carry anything big - or have lots of passengers.
I did have everything screwed to the boot floor - it looked a mess and then after following yours and Lenny's installations went for a false floor which freed up a bit of room.
The idea is that if I ever need to carry anything, the parcel shelf can be removed, along with the sub ( and the power capacitor can be slotted out (not quite as easy)) leaving the other four speakers in place.
Next thing to upgrade is the head unit I think.
BTW I haven't said but I've enjoyed reading your audio install thread.