1. Have you changed the UpgradeMode for SFBOnlyUser1 recently? I could be some delay or that you just need to restart the Teams client for TeamsOnlyUser1. The message that you get is kind of confusing since it says both that the user is a SfB user and that that user switched to Teams.
2. Since you can't send pictures between Skype and Teams that is what happends if a TeamsOnly user is signed into Skype, I think it the same if you try to share desktop.
3. Emojis are not supported between Teams and Skype so that is what happening. You just have to learn that (y) is thumb up and :o3 is a dog.
The issue right now with Skype Only mode, it's not complete. You can still launch Teams when it's set and if you launch it or have used it to send any messages from the SFBOnly user it's going to think that person is in Teams too, so that's I think why you get that message.
Skype Only mode when finished is supposed to not let the user start any chats etc.
That said, I'm going to assume you did access Teams from the SFBOnly user at some point? Even before you switched modes possibly?