1: Yes, the extra fee is so you can remove the 'powered by' link that appears in the footer of the gallery pages. People using branded licenses must keep the 'powered by' link, whereas people using unbranded licenses may remove the 'powered by' link.
2: With a license purchase comes one year of pro support and download access, after which time you can renew for $35 USD (subject to change) if you wish. The license itself is basically a 'forever use' license, subject to the terms of the license.
3: Yes, your forum style, as much as possible, is applied to the gallery.
Demo. Colors and images from the skin are used, but for example, if you have rounded corners in your forum, you would need to edit the PhotoPlog templates to add rounded corners to the gallery, as PhotoPlog has its own set of templates.