I have worked extensively with Joomla in the past. I also got some chance to work with Umbraco but that was only for one project. I think, since your project is in ASP.NET you should use Umbraco. This way you can use common nomenclature across your website and this CMS. Also, the knowledge of ASP.NET will come in handy if you need to have some customized behavior.
Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore, Dream. Discover.
I only used Umbraco among free CMS systems, but whenever I would recommend it depends on what do you want to use it for.
If you want it for small personal site, especially for yourself then it is good choice.
If you look for free solution to implement large project for your client, then it is neither free nor easy to customize.
Admin console is way too powerful with options to modify page templates, giving the client easy way to screw up developer's work. Also deployments are hell without paid addons if you have many page types.
"My software never has bugs. It just develops random features."
For a SWF, a flash file, you can send parameters to the file in querystring within the flash object html. YouTube does this to show more movie files at the end of movie.
In flash, you have a stage that can pan or slide multiple other stages in the main container, producing a slideshow effect.
I just posted a slideshow tip/trick, you can put flash objects in the child containers and they will pan, but if you have audio in the flash, you will have overlapping audio tracks. The script could be expanded with a ajax call to a web service to get data, and repopulate the child containers with dynamic flash objects, or simply just change the flash movie url.