Re Popcorn: I included the Popcorn article more because it seems to be permission/encouragement from TED to add layers to their videos (contrary to what their copyright restrictions suggest) more than to suggest is as a technology choice. Sorry that was not clear.
Re HTML5 vids: although HTML5 is being touted as the fully compatible alternative to flash, there is a problem getting them to play on iPhones (seems better on iPads. but not 100%). One site I read said this seems to have been improved in the most recent iPhone iOS but I'm running an older version so can't test it. The problem seems to be that iDevices don't play embedded videos inline. Hitting the big play button triggers the opening of the full screen Quicktime player & Apple seems to have locked out HTML5 like it did flash. What I see is the original video without your Questions, etc. I'm assuming you used some sort of embed code to get the video to display on the test page. I probably used an iframe and that in itself may be part of the problem. (I've just skimmed the articles found online as I'm not actually trying them out so am tossing out bits & pieces that were mentioned as causes & solutions.)
The folks at H5P are aware of this (as I read in their forum on video), but make no suggestions as to how to work around it. Unfortunately, despite the fact that they know their videos don't work on iPhones because of the HTML5 problem, they don't make that shortcoming clear to prospective users of their product. Unlike some companies which have developed their own HTML5 video/product players or player apps, H5P seems to have decided not to do that, yet still advertise their product as working on smartphones!
Suggested solutions found online include changes to the code &/or using a different video player on your website. The idea seems to be to turn off the big play button & add replace it with a set of controls that will allow the video to play inline (small screen on the page). I haven't checked Moodle forums yet to see if they already have an alternative video player plugin that works, but that might be an avenue to try. Can you download your H5P video & upload it to YouTube? They have apparently fixed their player so HTML5 vids play universally. That might work.
I've been looking for a video player app to install, but no joy there. The fixes all seem to have to do with adjustments to the website not on the iDevice.
Wish I had better news & that I knew more about code. You may have to develop a parallel experience on a different linked page that participants can jump to if the first 'test' fails on whatever computer or device they're using. This could be a good model of how to make online activities available to all and there may be someone in the next workshop who knows the solution :-)
That's all I have for now & coffee some time would be fun. I'll see what the Moodlers have to say.
My video teacher would have called this a good lesson in the value of testing the production path before investing a lot of time developing a full end product. He used to have is do 30 sec. takes of anything & test all functions as the first activity in any lesson so we wouldn't be tearing our hair out when an assignment was due ... Wise man!