Hi Daniel,
Thanks for sharing the results of your interesting investigation.
1. If you only changed video output picture size (1920 x 1080 vs 1920 x 1280) it didn't change original show aspect ratio and thus it added black strips when original aspect ration was different with output size. So the effective picture content (sum of pixels) didn't change. So you need to change the project aspect ratio and output picture size together and reencode video again.
2. Interesting result. I think it may depends on the content - animated images or video content. Also probably Quality = 100 can be excess value for animated static images. I'll repeat your exploring later.
3. Yes, the contant quality compression is a great mode and it produces excellent results in terms of a quality and file size. I always recommend to use this mode, especially for static images or animated images.
4. Motion Blur. As far as I know, almost all apps that create this effect (including PTE AV Studio) render intermediate frames, which significantly slows down the video encoding time. For example, if you set FPS = 50, PTE generates 1000 frames per second (50 x 20) and blending these extra 20 frames into one final frame. PTE doesn't perform analysis for the content.
Motion Blur option effectively applies only to animated PTE objects, not for embedded video clips.