The extension to do any controls advertises itself as Roku TV (TCL on Mine) this needs to be added to the Harmony app as a device. If you enabled device correctly in the extension then scan for WiFi devices in the Harmony app it should see it and you add it. Then create an activity using streamer, amp etc and include the Roku TV it needs to be the device that controls the media playback when setting it up in the activity. If you follow the instructions properly for setting up the extension it covers all this. Look at this as it covers it all.
If you want Alexa to play music it won’t be able to. It will be able to play pause skip etc but it won’t be able to play specific tracks or albums so I don’t think your wife will use it as you still need Roon app to play the tracks albums you want as it can’t search the library.
This extension allows the Roon app to control volume on devices it normally can’t, trigger the whole activity so turns everything on/off and allows a hardware remote to control simple playback of Roon. Alexa can be added to Harmony to then add simple voice commands such as turn on TV of hifi etc, play music, stop music.