@CordulaGöke - As I stated in my reply it appears that you are running into 2 issues: (1) the TOC/legend is not showing changes you make in the Symbology pane and (2) when you export a layout the legend in the exported file does not match what you see in the layout view. You have found a work around to issue (2) by using ArcPy to export.
Please let me know if I have misunderstood your posts.
Unfortunately, given the current information I have to work with, I cannot reproduce either issue with my own steps/data. Reproducible steps to recreate the problem are needed in order for the programmer to debug the code and come up with a solution. Seeing a problem posted on Community is not enough. We have to be able to reproduce the problem. Otherwise, we are just guessing.
Community is not a bug tracking system. Sometimes an issue reported on Community is easy to reproduce and an internal bug can be logged. However, many issues require a better and deeper understanding of the details involved in the issue (what are the steps taken before encountering the issue, what is the data being used, what renderer is being used for the layer, is there scale dependencies involved, etc... there can be many variables).
I'm sorry that you have had a frustrating experience with support, but they are better equipped to process bugs and I would encourage you to contact them concerning these issues.
Tom