I checked the Forum Features Guide and here is the advice I saw for a quote within a quote:
Blockquotes can be nested. “>>” indicates a quote within a quote.
However, when you use the quote button it doesn’t use “>”. To make a quote within a quote I added a single “>” in front of the quoted text. My initial idea was that it was wrong to edit the text that Discourse selected after hitting the quote button. I think it’s more clear this way though.
I know I’ve been warned on misquoting in the past. Sorry for making additional work for you.