Try adding:

margin-left: -15px;

... to the .sv-nav-page-label class in the 'simpleviewer.css' file.

Your gallery looks centered when I view it in browsers on my PC.
One method of horizontally centering the 'sv-container' <div> would be to replace:

<div id="sv-container"></div>

... with:

<div id="sv-container" style="margin: 0 auto;"></div>

It is not possible to position the Button Bar in the Mobile Player using preconfigured options.
The Button Bar Options (such as buttonBarPosition) is available to the Flash Player only.
You could try changing the values of the .cls-topmenu class in the 'simpleviewer.css' file.
However, please note that modification of the 'simpleviewer.css' file is not supported. I do not know what knock-on effects changing any of the above values may have to the layout of your gallery.

The ability to create an RSS feed is not part of the functionality of svBuilder.
The only plugin which I am aware of that would be able to generate a 'media.rss' file for the images in your gallery whilst the gallery is being created is the SimpleViewer Skin for jAlbum. jAlbum can generate a 'media.rss' file for each folder/subfolder in your album's input structure. In order to use this functionality, go to 'Album -> Settings -> Advanced -> Metadata' (from jAlbum's drop-down menu at the top of the application) and select the 'Generate Media RSS feed' checkbox.