1. I would suggest reading all the announcements on the various releases in the announcements forum. There are way too many changes to list.
2. There is no evaluation period since you need access to the code to get it. You already own a license. You would either have access to download the new code or not. You may purchase a members area renewal if your expired. I would highly suggest upgrading just because of all the fixed security issues.
3. You can check out photopost at a number of sites running the software. There have been tons of new features and changes all the way back from 4.6 . The interface has changed slightly depending on the features added etc.