I think it should be enabled by default for all. You can disable it by clicking the left icon on the toolbar:
It’s a WYSIWYG[1] editor, so when you write bold, italic, lists, quotes, links, post images, etc, it should appear in the composer window without the need for the former preview on a side panel.
Since there’s now a visual editor, the “old” markdown editor can be considered more like a code editor, and monospace fonts are the standard in code editors.
I don’t think so. I’m not sure this has even been requested on the official support forum. The current width is about the same as how a post is displayed in a topic, so it reflects the final result as accurately as possible.