At the moment, when you are on the wikitext mode, you can click on a link to preview your changes.
When you are on preview, you don't have any link to switch back. You have to guess that you must click on the pen dropdown menu. That that's two clicks instead of one.
A possible way to solve that would be to have a link to switch back, only displayed if you have used wikitext editor or if wikitext editor is your first editor. That would be included in a sentence saying something like "You are on the visual mode to preview. You can directly edit your text, or [switch to the wikitext editor]."