No license manager exist.. It is a honor based license model..
No, as long as you have enough ThinApp licenses to cover your deployed environment you should be fine..
Yes, the package will expire. What you need to do is to either rebuild the project or relink the packages using the new paid license key..
More details: http://blogs.vmware.com/thinapp/2008/12/how-to-chg-lic.html
Benefit are that the project is without license.. So any tweaks you do will be full reusable when you have your real license.