after updating Simpleviewer a bunch of the galleries have now vanished
I notice that the SimpleViewer gallery on the page you provided a link to has empty strings for the width and height of the gallery.
I have not seen this before or had a user report this before so I do not know what might be causing it. Also, there is no setting within WP-SimpleViewer that would modify multiple galleries in a single action and upgrading the plugin does not modify the existing XML files.
However, if you go to the 'SimpleViewer -> Manage Galleries' page, edit Gallery #49 and insert a width and height before saving the settings, your gallery should display on your page.
If you have many galleries that are missing width and height entries, you could fix them all by doing the following:
(1) Download all XML files within the 'wp-content/uploads/' folder
(2) Open all the XML files in a text editor which can handle many files at once (such as Notepad++)
(3) Do a global search for e_g_width="" (across all files) and replace all instances of the string with e_g_width="100%"
(4) Do likewise for e_g_height="", replacing all instances of the string with e_g_height="600px"
(5) Save all the files
(6) Upload all the XML files to their original location ('wp-content/uploads/')
Why is this happening? Is it because the galleries were created before WordPress made so many gallery changes in 3.5?
No. This is not the problem. Although WordPress introduced many changes in the Media module in v3.5, behind the scenes, things have not changed as far as WP-SimpleViewer is concerned (images attached to the post will be displayed in the gallery). The new Media interface just changes the way that users attach images to the post.
As I recreate the galleries I'm getting xml error unless I first create a WordPress native gallery of images already uploaded to the page. Then add the simpleviewer one.
There should be no need to create a WordPress gallery first. You should be able to create a SimpleViewer gallery by following the WordPress v3.5 instructions here.
SimpleViewer Support Team