Just a quick follow-up
I got the API running - it’s quite straight forward if you use the tutorial of the post above.
Now I can answer the question 4 
The update interval of the API is: 5 Minutes! I really did hope it would be a bit faster, but hey - it’s a quick way to read the Solis without any additional hardware and set-up/coding. You can actually spend hours to set up the visualization because there is a shitload of information …
Yea - it’s in german and it’s not nice, but I’m on a learning curve. Started using Home Assistant just 5 days ago …
It’s kinda funny, because once I got the setup it began snowing and now there is a fine layer of snow on my solar panels. My peak output is 68W 
But I have faith in global warming …