Low Poly shoes with major holes

Parts of what should be the white/red top of the shoe is mapped to the small, grey area.

For the flipped parts, select just those (you can select by face, one by one) and then recalculate and/or flip direction until they're all flipped in the right direction. Right now bits and bobs of what should be in the white area is actually mapped in the grey area, so it will look a bit weird when you try to flip it.

Then you'll have to remap the areas so they are in the right spot. From there you should be able to proceed as normal, but you may want to fix one shoe first and then flip it over when it's done.

There appears to be a double layer of polys all over (seems to be a separate layer in the group - select with P and you should get most of it, but you may need to do some cleanup in the sole area). There's still at least one flipped face in the problem area, which likely is causing some problems - there's actually 4-5, but some might not be needed and can probably be deleted once you've sorted out which ones you need.

There are also some extra polys on the inside of the shoe you don't need, especially toward the sole.

If you temporarily hide the foot and look from the inside of the shoe, it's easier to see.

I think there might have been some extra duplicating going on, because I found a whole extra shoe underneath the mesh, and 5 extra straps under the right foot strap, so you may want to check that out, or the polycount is going to be much higher. Did a bit of quick tinkering to test what removing these polys would do, and a single shoe went from around 1500 to 500 polys.

The easiest way around it was honestly to mark each face one by one, separate it out and make sure you got it all (make sure the legs stay in the original group), remove what's left apart from the legs, and then join the two groups back together (tedious, but when there's a mesh layer directly underneath you risk getting parts of it selected, which I suspect could be what has happened here).

When you duplicate, it can sometimes be a good idea to duplicate then separate the duplicated item, and join back together when you've mirrored/edited. I've noticed Blender has a tendency to otherwise duplicate items a few extra times if you're not careful about your steps, and then you could be stuck with double/triple the polycount without knowing why.