So I've been trying to figure out how to get my chromecast to connect to my phones hotspot and then how to screen mirror from that same phone running the hotspot. Well apparently this feature has been added to Google Play Services starting with version 11.5.03 beta. I've installed 11.5.03 beta and 11.5.07 beta and they both support the functionality. I got the play service update apk from www.apkmirror.com just a google search. From what I read I guess you can sign up for the google beta program for Google Play Services Beta updates. Anyway I'm always taking info from this site glad to share some for once. Hopefully this isn't just news to me 