Since using lenses with different focal lengths affects the amount of background that we can see and some of the backgrounds will be more in focus with a further back focal length, is it possible to combine images taken with different focal lengths to lower the background blur of a wider angle shot?
Yes, it's a technique called focus stacking (https://en.wikipedia.org/wiki/Focus_stacking). Basically, you align the images, then manually (or automatically using edge detection or frequency analysis) choose which areas are in focus, and finally combine the different in focus sections to create the final image.
This series of images are great examples of Dolly zoom (https://www.wikiwand.com/en/Dolly_zoom). The first application of it in filming is Hitchcock's Vertigo (https://www.youtube.com/watch?v=u5JBlwlnJX0 @ 0:24)
Is there particularly any reason to use shorter focal lengths in an image like this? I'm struggling to see the purpose for short lenses for human subjects.