I understand where you're coming from, and I'm sorry to hear your experience isn't what you expected.
to answer your question, I believe it should be possible to modify the page properties in a batch using the API. there is a simple example of using the API to modify a page here: http://www.sitefinity.com/40/help/developers-guide/sitefinity-essentials-pages-modifying-page-properties.html
You could retrieve all the pages, then use a string replace to delete all double dashes ("--') and the ending string ("-aspx") from the url-name property of the page.
I do recommmend that you backup the database, as well as simply outputting the resulted change (instead of saving it directly) so you can see what's happening before you commit anything.
However, this hopefully should get you on the right track to fixing those urls, please let me know if you have any questions or encounter any issues.
hope this is helpful!