Just successfully upgraded NextGen to 2.0.40 on the site using the imagebrowser over which I initially started this thread. Everything is working with one substantial glitch. My custom 'nggallery.css' stylesheet keeps getting over-ridden by the default imagebrowser stylesheet. If I reload the browser, then the custom style sheet is properly referenced. But randomly(?), the image browser stylesheet takes over again when navigating through images. It behaves the same whether I'm using legacy shortcode or the current gallery code.
Was able to select in NextGen settings.
Customized template here:
Was able to select in NextGen image browser settings.
When I disable this file everything works perfectly:
I suppose another consideration is that my custom imagebrowser template is the one I used with pre 2.x NextGen.