Hi Erik
I walked through the same steps you describe but my experience wasn't the same.
Editing a page with some existing HTML in the content area I pressed the orange Content Layout Editor button.
That took me to the Layouts content editor, as shown in the first screenshot. It looks similar to editing Layouts previously, but I am only editing the content area for this page. I can still add rows with additional content etc., and the existing content is in a Visual Editor cell.
After saving that, when I return to the page editor I see screenshot 2, showing me that the page content is being designed with Layouts.
If I choose to stop using Layouts I'm asked what content I want, and when I choose the content from the last version before using Layouts it reverts me back to where I started, with the original content in the page editor.
* Update *
I'll leave the above intact, as it was my initial experience. But when going back into edit the page a second time and repeating the above, I did lose the initial content.
So then I repeated the steps over and over, lots of times, with variations, but couldn't get it to lose the content again. That is, I witnessed the problem once, but then couldn't reproduce it.
Are you seeing this problem every time you try using the Content Editor, or did you just notice it once?
Regarding the CSS issue, I'm not sure I follow. Are you talking about adding custom classes to cells in the child Layout? Or CSS rules you have globally added at Toolset > Layouts CSS and JS?
(If I need access to your site I can set a private reply where you can share details and only I will be able to see them.)