If someone stumbled upon this thread: I have solved it with
.CodeMirror-lines, .CodeMirror-selected, .markdown-preview-section{ max-width: 620px; max-width: 65ch; margin: 0 auto; }
(Ch is a newer CSS property that put the max width per character. The 620px in this example is a fallback.)
Optimal reading length is between 50-75 characters. Depending on the type, width, context, etc.
Till now nothing really broke. Panes work and scrollbar is on the far right.