Removing extra surface in imported solid

Hello,

I have an imported solid that has an extra solid portion in a thru hole that I'm trying to get rid of. I tried Import Diagnostics, that couldn't fix it. I tried converting the hole to a sketch and doing an extruded cut (with sketch offset, plane offset, etc), no luck, says the operation failed due to geometric conditions. I tried filling the hole in so I could possibly redo the hole, but got the same error. I even tried the hole wizard, but it says the intended hole doesn't intersect the model, even though I can see the transparent yellow preview solid intersecting the piece I'm trying to delete. I've attached the model, along with a screenshot pointing out the problem holes.  Let me know if you guys need any more info. Thanks!