I know this is an old post, but I had a very similar issue and I was not satisfied with your final 'resolution'.
In the mean time I discovered something that actually solved the issue for me without any of the mentioned workaround hacks, so I wanted to share this with the community for other people landing on this post.
In step 2 of the metadata wizard, make sure that in the bottom right with "Please select schema" you have selected the correct excel sheet.
My first sheet has a lot fewer columns than the sheet I actually wanted to use as input.
So by having the first sheet selected, the wizards does not allow you to select the correct amount of columns in step 3.