The fact is, the slider worked with the purchased version of your theme. You can also see this from the fact that changes have been made to it. Some updates later it won't work anymore.
Your developer claims that the slider never worked. But how could I make changes to the slider back then? ...
So either you don't want to or you can't help with this problem. That's ok, I accepted that and won't book another support subscription anymore and will change the theme in the long run. I have never experienced anything like this before.
Is there at least one tip from the developer? Or can he say why the slider doesn't work? The statement it doesn't work, doesn't bring me much.
Since the site is live I can't just switch to another theme! However, I can confirm that of over 80 active WordPress installations on my hosting server, only the installation with your theme will cause problems with the Slider Revolution.