A:AnswerYes, with two things to consider. I use this with DaVinci Resolve and Final Cut Pro X and it definitely helps. The two things to consider are 1) does the software support it and 2) the perforrnance boost you get will depend on the GPU you put in there.
A:AnswerFor a MacBook, you need to use an AMD graphics card due to driver support. The fastest card available at this moment is the 5700 XT. New cards are being announced in October that will be much faster.
A:AnswerYes you can. However, depending on several factors such as the games you are running, settings, resolution, and if you are using an internal or external display you may or may not see a benefit compared to the performance of the RTX 2070 Max-Q in your Blade 15.
A:AnswerYes, IF that iMac is thunderbolt 3 equipped and capable of running 10.13.4 or later.
https://support.apple.com/en-us/HT208544
No, if it is an earlier iMac. I believe the cut off is somewhere around 2017.