Just this very second I figured out what was wrong: It was the ‘frame size’ under Sequence > Settings > General. It is at low resolution by default and whatever final resolution I chose for the exporting it was always managing my photographs under that other lower quality. So what I did was make it larger (closer to HD, in my case 1620×1080, to keep the 3:2 aspect ratio of the original pictures) and this immediately made the final video top quality.
As for the aspect ratio changing after exporting all I had to do was select ‘Square’ in Sequence > Settings > General > Pixel Aspect Ratio and then reemport every single photo into the timeline and rendering them all. Now the final video is of great quality and aspect ratio, choosing the H264 codec, .mov format, and export size at Current.
I hope this helps someone caught in the same situation! 🙂