Dear Franz,
Concerning the sun shining on a piano... I would not recommend it for two reasons. Changes in temperature are never good for the piano. There's always some humidity in the instrument and it's better to keep that stable. If something is not right with you piano, it will also be easier to detect, to diagnose the problem if the instrument has been kept in stable conditions. Another reason why the sun is bad, I experienced it with my piano, is the change in colour. The brown, in my case, is less warm, intense as it would be if I had kept it out of the sunlight. So, no sun, stable climat, as less moving as possible, regularly tuning and lots of gentleness and love. The thing will give it back to you.
Have a nice day.