I really can not speak about 5.62 thats been alot of years ago
I understand your sad about your integration issue but it is a simply point. If you integrate with a forum your using that forums users. The only way Photopost uses its users table for a standalone install for authentication otherwise authentication comes from a forum.
Your web host might be able to assist you with your server setup and errors. I am unaware of any issues with Photopost with various different servers. Gateway timeout issues are usually a server setting from what the net says.
As far as vbGallery there is no upgrade path to Pro as thats a totally separate product you would need to install Pro and then import vbgallery content to Pro. There are tools in the imports forum to import vbGallery to Pro.