Matthias, compression is supported in a different way. You can select items in folder view and compress them to a new archive. See folder view context menu for the relevant commands. Edited files will indeed not go back to an existing archive. But editing still makes sense because you can create a new archive from the files in the temporary folders.
I think it has been a bit of an unfortunate decision to offer the option to share components of an installed copy of the 7-Zip application. Users experience is that after they update 7-Zip, WinMerge archive support is broken. There is a fix, but it does not help those who aren't aware of it.
Recent 7-Zip has a few conceptual changes causing some headache. I had seemingly working code about a year ago, but no time to hang on to it.