Personally, I highly prefer vertical sorted lists because of the following reasons:
-it's standard, every book uses vertical indexes.
-similar names are much closer to each other in pixel distance, requiring less mouse movement and thus making it easier to see the hovering mouse thumbs.
-the letters/characters in each name a fully aligned, making it easier to see the differences in names.
One drawback is that when you're viewing a large vertical sorted lists with multiple columns, and you want to find some specific name, than you don't know whether you have to scroll down in a column, or to scroll up in the next column.
Maybe we can make an option to switch between both sorting methods.
By default, Chrome and Firefox show vertical sorting while Internet Explorer shows horizontal sorting.
For the Duplicated model reporting links, you're free to use any method that you prefer. The link is meant to quickly tag a model as duplicated, when you don't know model's official name. Especially handy for those model pages from TeenDreams, which uses a lot self made up model names. The duplicated model links are not meant to tag model pages that use official names (and have a lot of websites mapped), like
http://www.indexxx.com/models/28749/ann-marie-la-sante/ , because it's difficult to find unofficial name pages given an official name, to tagging that pages doesn't help much. But it's a lot easier to find an official name given an unofficial name (+ photo), although it still can be difficult.
Great to hear that you want to include Birthdate and other bio info!
About DVDs/movies, this is also something that should be added to the model pages in the way sets are added. Like iafd.com, but with photos. Not sure yet whether this should be done using an intermediate site, like hotmovies.com, or to use data from the producers directly.