This is one followup, discovered when testing MDL-35172. From that issue:

...there is one simple case where it seems to be borking the editor and it's when you add one line having ALL elements invalid (dupe or incorrect).

So for example, simply by adding one 4th line with "fontselect" it causes the editor not to be rendered (it is a dupe from first line). But if you add one 4th line with "fontselect, blockquote", it works perfectly, the dupe (fontselect) is ignored and the blockquote is shown.

And that is, all elements (in a line) invalid lead to editor not rendered. Just guessing if you could pre-detect that or no (certainly you don't know in advance all the possible valid elements, or yes?)

Ciao