I don't know of a way to turn on Slideshow mode via an Opus command... I don't think you can.
However, you could send the hotkey that the viewerpane uses (<Ctrl + h>) to enable Slideshow mode via an external scripting tool like AutoHotkey. And to be able to view all images in a folder structure (though not just the "selected folders), you can use Opus' Flatview mode...
Here's an example:
This button goes to a particular directory, turns on Flatview, opens the viewer to show whatever images, then launches a compiled AutoHotkey script.
If you really want to only operate against "selected" folders, then I think you'll have to first add the selected folders to an Opus File Collection, and then switch to that collection before running the "Show" command in the button... Anyhow, this is the basics of what you could do to get what you want. There may be other simpler ways that I'm overlooking?
FWIW, the AutoHotkey installer provides a utility (called ahk2exe.exe) which compiles .ahk scripts into executables. I've setup my own filetype for .ahk files which I've added an Opus context menu entry to which runs: