Hi Mark, I've attached here a finished version - I used the steps I described above to select and then delete the faces for the inner portion.

Then I drew in a dividing line from the midpoint of the top edge to the midpoint of the bottom arc, which is not an exact vertical line though because the shape is slightly rotated. Then using that line I used the Edit > Trim command to slice the object into 2 half pieces, deleted the half with the messy area, then mirrored the good half and then joined the mirrored pieces together to make a solid.

Hopefully this is the result you were trying to get.

- Michael