LouisV,
Lots of points, not too many real questions that I might try to address. Basically it sounds like you are confusing how integrated laptop display graphics vs desktop graphics work (item 3, 4, 5, 6). There is not a software switching approach like Nvidia Optimus Technology (laptop) on a desktop. The major functionality of that technology is to extend battery life by only using appropriate display which is not pertinent for desktops. The GPU and/or iGPU is essentially controlled in the BIOS on a desktop if you want to only use one or the other. Yes both can be functional (drive a monitor off either or both). Many laptop suppliers also offer ability to turn off iGPU on many models (but not all) and only use discrete GPU via BIOS as well. Dell laptops rather compliant with the feature, ASUS ROG typically not so compliant so what one buys does make a difference with how you intend to use.
No I would not recommend nor suggest any AMD GPU with PD.
Depends what you mean by "more speed" in item 3. More than likely the best CPU on your budget will provide the best overall editing speed benefit in PD. However, if by speed you only mean encoding speed, then depending on budget some 900 or 10 series GeForce would be a significant advantage over most CPU’s and software encoding with PD. Although with hardware encoding comes some encoding anomalies with PD, most of these are predominately in the Create Disc area. Since you’ve read practically all the topics NVidia-PD I’m sure you are well versed in these issues.
I’ve not experienced a issue with any desktop Nvidia GPU and driver with PD that a correction was to resort to the Windows VGA driver, maybe someone else has that experience and can share with you. I say that based on my use of many Nvidia GPU’s (GeForce 8800, 9800, 210, 470, 580, 650, 750Ti, 960, 970, 1070, Quadro 2000, K620, K6000) to name a few.
Good luck on your new platform.
Jeff