I am running Quill in an HTMLViewer. To integrate this with a Xojo app was some amount of work.
2) Because the events available (StatusChanged and TitleChanged) that allow getting results back from the HTMLViewer were not guaranteed across platforms, and neither was the amount of data unlimited, I ended up implementing a WebSocket server to allow easier data exchange.
3) I had trouble with the Windows renderer. Firstly, what worked OK under macOS didn't work in an HTMLViewer under Windows until I ensured that (a) IE11 was installed, and that the HTML page loaded into the Viewer demanded that the IE11 renderer rendered in IE11 mode, and not its default IE7 mode. Then it worked OK.
So it works OK but not perfectly, there are some things that could be improved. But what I needed was a way for a user to edit some rich text (bold etc), add images, add links, and for me to be able to get an HTML representation of that.