If your notebook does not meet Apple`s criteria then there is little you can do. Apples about one thing $$$ and wants to minimise it`s loses, hence why they ignore their customers, albeit ever so politley until faced with court action. The dGPU saga with the MBP is just getting old, with so many versions of the MBP afflicted over the years. I find now that I am looking for a new portable with dGPU, equally the current 15" rMBP is not on the list, nor will the 2016 model if Apple continues to push consumer grade silicon to the extreme of it`s thermal limits.
Now with the Windows OEM`s catching up on "thin & light" with more powerful cooler running dGPU`s and Apple`s rather humorous pricing for what can be construed as a flawed design, spending $2.5K on the rMBP seems a little unwise, especially given the history. I also believe that Apple will drop the dGPU on their portables as soon as they possibly can, as it flies in the face of Apple`s design ethos.
Q-6
It seems that Dell´s XP15 series are a very good choice in every point. The new (old-fashioned) line of Thinkpads either.
Especially as the recent OSX-Version since 10.8.5 (and IOS versions also) are more and more "thinner" = full of bugs either...