Hello,
I looked into the project and here is what I can tell:
* unrar library which you suggested in pdf only works with rar files, so for .zip files I'd use another library, like miniz for simplicity (libarchive seems to not have an easy way to get checksum)
* I'm not sure how the app can hand the windows explorer. When I had to add a context menu last time, I simply added registry keys which add context menu for given file extension. The explorer starts the application, like any other app, and it does not block the explorer.exe. For large archives, I could simply show, 'please wait' dialog, so I do not see the problem here
EDIT: sincei t seems important to you, the zip file library I'd use is in public domain
EDIT: I was able to extract information from both zip and rar files. Please let me know if you are interested