That is a strange case. Flatiron has no additional requirements and when you can run UV-Packer you should also be able to run Flatiron without problems.
Do you get any additional information? The error message 'failed to load DLL' usually also mentions the name of the dll.
Are you using the most recent version? Try re-downloading and reinstalling flatiron from
http://flatiron.3d-plugin.com/download.
Your 2nd question:
No, Flatiron does not contain UV-Packers functionality. While UV-Packer repacks previously created UV charts, Flatiron generates its own unwrap as part of its full scene texture baking functionality.