Read over the product information page onhttp://www.smfhacks.com/smf-gallery-pro.php
There is no manual for the pro edition.
The product is stable as there is hundreds of people using Gallery Pro if it was not stable people would not use it.
As for money back that is normal for digital products and most major scripts have a zero refund policy since it is a digital product and it is hard to say wether someone would be satisfied with the product or not. If the lite edition is working well then try the pro edition if does not fit your needs then maybe look into freeware gallery scripts.