How to edit sloppy 3d mesh

I’ve got a 3d model that is derived from a photogram from RealityCapture and converted to SKP using Transmutr. It seems to open in SU fine, though it’s not clean. I’ve deleted a bunch of it to make it more manageable but of course that leaves loose ends.
I want to throw carve a shape from it, and then shell (add thickness) to that. How can I cut out a shape, add a smooth edge, and thicken it (shell, loft?)? Thanks for any help
ft0619b.skp (12.7 MB)

The model is very small for the tiny geometry. Perhaps if you can import the file into SketchUp at a much large scale, say meters, it would require less cleanup.

Are you still using SketchUp 2018 as indicated by your forum profile?

oops no I have SU 2024 Pro. But I am not very good at it.
I tried re-converting it with Transmutr, which made it much bigger (0.25m vs. 0.25") but it has the same amount of loose ends hanging off and such. I can trim it down to rough shape, but how would I go about give it smooth edges and thickness? I need it to be the size of a human foot (can it 12", though really only the front 1/2), and maybe 1/4" thick. What’s the proper tools to do that with?