I was tired of Excel opening additional documents within the same instance, so I looked into how to force it to open a new instance for each file. This answer worked great.
However, I then got an error every time I tried to open a file from Explorer instead of going to Open from within Excel. This post helped me correct that by editing some Excel registry entries. I did the same for CSV as was listed for the Sheet.# entries in that blog post.
Now I'm just left with one lingering issue - I can't open an Excel attachment directly from Outlook. It opens Excel, but no document is open and no error message is displayed. Is there a fix for this?