I cannot answer all of your questions, but I can say that if you are trying for realistic renders, under Render Settings you select the Nvidia GeForce option. CPU should almost never be selected.

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System: Alienware 17 laptop: Windows 10 Pro, 32GB RAM | Intel Core i7-6700HQ (Quad-Core, 6MB Cache 3.5GHz) | Onboard Video: NVIDIA GeForce GTX 970M 3GB GDDR5 1028 CUDA Cores | Alienware Graphics Amplifier: GeForce GTX 980 ti 6GB (006G-P4-4996-KR)  2816 CUDA Cores

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