Hey there, it’s me again. Your feedback last time made think a lot and I reconsidered some decisions, which in the end helped a lot to finish my extensions now. Once again thanks a lot! 
I decided to go with Ajax instead of WebSockets, which made it possible to develop multiple extensions using the same backend. For the security issue, it was enough for me to obtain a free license from CertBot and replace the testing.key testing.cert with the real one.
Here is an up-to-date screenshot of all the three final extensions:
Once approved and whitelisted for a group of chess broadcasters, I will perform a study of 45 minutes, where everyone will play CrowdChess using those extensions. Regarding the collected data of Twitch-user, I would like to know what are the necessary steps not to run in any privacy policy issues. Do I need to contact the Twitch Data Science-Team and ask for their approval? Maybe you know more about this, help is much appreciated! 
And now for everyone: In case you like chess and would like to try out this new experience, where the whole audience plays together and alters the content of the stream, you are more than welcome to participate. Date and time will follow…