Regarding the iframe it's a bit more complicated to explain.
We don't want users to embed outside-content anywhere because it could break the step-by-step template of a tutorial.
But it will be possible on the right part of a step only.
We are working on it (see task #4 on the product backlog).
What we thought of is a field in which the user can past the URL of a third-party-site (like Youtube). After saving the tutorial, Wikifab will automatically convert that URL into a viewer.