Is it possible for me to edit the HTML pane?
For example suppose I want to insert working embed code to show a Youtube of Vimeo video inside a note. Is it possible for me to edit the raw HTML in the note to do this?
In short, No. That’s not how the process works, as the HTML (preview) is there to let you check the content/templates you are using are generating the correct code. The Preview tab goes a stage further and shows you the HTML as rendered in a web browser (I guess using Apple’s web tech).
You can export the page and edit the exported file’s HTML. If unfamiliar with HTML, that can be a good way to figure out what your code in Tinderbox should generate.
In Tinderbox you can insert the HTML code for the video into a template into a note’s $Text. The latter will mean you see either/both ‘raw’ HTML or Tinderbox ^export^ code in notes text when viewed in Tinderbox.
If not sure how to do that, it would be helpful to know exactly what you are trying to embed. Giving generalised open-ended help on this is means lots of “yes, but…” follow ups. It’s better to start with a finite example and then generalise out from there.
Also, now @satikusala’s video tutorials are rolling, I’ve little doubt this sort of issue—video embedding—will get covered in due course.
HTH 
OK thanks - two examples:
(1) Suppose I want to embed a Vimeo video like this in my note - both for my own viewing and also to work when I later export it as HTML on a static website:
(2) Suppose I want to embed a PDF which is viewable inside a note - either hosted on Dropbox like this or via some other method:
OK never mind - this is way simpler than I realized - as you said, I can just enter regular HTML in the Text pane and it appears in Preview - I did not realize that - that has much potential
Is there an attribute for me to choose Text vs Preview as the default Pane on a per-note basis?
No.