This text though, is a collapsible inside a collapsible, and can contain anything you want it to. Unfortunately it means you have to write this in raw html (or have it otherwise converted), and need to have the necessary CSS included too to make sure everything looks right, so its not an ideal solution for minor things. If your entire skip hangs on having nested collapsibles though, the option is available.
You can have raw text in here too, woo!
And quote blocks!
If you really don't want to write your article in raw html (which is understandable, its a huge faff), you can cheat a little. Write your article as normal (without your nested collapsibles). Once its done, most browsers will give you the option to view the source code for a webpage, so load up your draft, view the source and copy the raw source code for your page. You can then use that instead of having to write the entire thing in HTML yourself. Obviously this requires a least SOME knowledge of how html/css works, so it isn't a catch-all solution (you'll also need to manually copy over the CSS your page uses too). So, the reality is, unless you have some understanding of how all this stuff works, your best bet is to not use nested collapsibles.
You can see how all this is working internally by clicking the History link at the bottom, and then the View Source link (the S) for the latest revision.