Welcome to the forum.
Read your thread and found it interesting as you are doing things slightly different.
I am sure that you have done your home work but few things to suggest:
1- HU: I'd straight away eliminate JVC HU (seriously, can they make a good HU). Looked at Kenwood HU very quickly and I do not think it comes with auto tuning of DSP and EQ as apposed to 80PRS. And if I am not wrong, you will have to manually enter centrain readings in Kenwood HU to get the optimal results.
For me 80PRS or P99RS are good HUs. Can you not stretch your budget and go for 80PRS?
2- Harness:
You may wish to consider
this. This is all in one solution.
You may wish to relocate your HU in the trunk, in case if your car goes to dealer for any upgrade.
Also you may require harness from amps to you OEM harness --> speakers.
I am sure these should not be a problem for you if you do your home work properly.
3- Drivers: Can you kindly post the link to MB Quart custom fit co-axials?
4- Sub in the boot:
You may wish to read reviews on the subs fitted in that storage area.
Also, do you mind sharing your previous car and previous installation (s)? You seems experienced chap...
False floor seems a good idea, but have you done it before? Are you going to do it yourself or somebody else is doing it?
Your amps selection is brilliant. I am not sure about their sizes, but could they go to the space where OEM CD changer sits?
As Mob17 suggested, you may wish to read other sub installation methods. You may ask your guy in sheffield to build a corner enclosure as from my personal experience it works better compared to other approaches, except IB install.
IB install is very unique and worked out best so far for many of us. You may wish to read Mob17 and Kaigoss IB installation which are more OEM friendly than my IB install.
If you go for corner enclosure or IB install, then you can use that storage to fit your amplifiers.
What about under seats? They are the weakest link / point in BMW installation...so think very carefully about what you are going to have it there.
Looking forward to read your response / progress.