Can Alexa create groups of devices/endpoints if I provide some additional information in the Alexa.Discovery.Response?
Well there are groups and groups. If you want them to appear in the Alexa app as a group, then indeed no. But if just want to use a group name by voice, you can fudge it. During discovery you can add a SceneController that when triggered will run a loop in your Lambda over an array of devices (so you still get it in the app, but under scenes). I use this approach with TemperatureController to set a group of rooms to a common temperature. Main thing might be how to name the group. I added a UI field locally that allows me to specify an array of group names that endpoints belong to. Otherwise, some smart pattern matching with regexes will go a long way towards guessing a sensible group name for your user. And if they don't like your guess, it can be edited by the user in the Alexa app.