The use case is that a user can paste possibly formatted text from a Word document or e-mail-App. Preserving structure (line breaks and paragraphs) is more important in this case than preserving the format, since most of our users wouldn’t even perceive common body copy from a document as particularly formatted. So we will just use your textarea widget.
EDIT: Sorry for posting double, but it seems not so reproducible anymore in Firefox (see this fiddle: http://jsfiddle.net/yxuesquz/2/). Pasting the blue text below results in plain text pasting even in html-Mode in Firefox, but the possible bug still exists in Safari (9.0.3).