Hello Alexandra and welcome to the community here, and asking questions is what we love to see here.
I am a huge fan of H5P, it would be great to learn more about the project you are working on and even about the types of H5P you are creating. Also you will find a number of conversations here by either searching on H5P or using tags.
For alternatives to annotating in Pressbooks, one thing to consider is that for the most part Pressbooks are a hosted service, so you cannot add other tools to the platforms. The advantage of Hypothes.is is the social aspects, that you can see and interact with others (there are plenty of annotation tools if it’s just you annotating).
And yes, if you are using the free service, the only way to make activity private for classes is to set up groups. It can work, but relies on students remembering to choose their group first.
There are options to better manage it if you have Hypothes.is connected to a course management system like Canvas, where students access annotating via that login and not the public one. But that’s not an option for all schools (it costs $$).
To me I’d want students to maybe take on public annotation as a way of adding to public knowledge, like the successful projects of students working on Wikipedia articles. Maybe it is something that can be scaffolded, but also depends on the way annotating assignments are created.
For H5P, when saved to PDF, the content is replaced by a box indicating that it is a placeholder, and offering a URL that can be used to access the H5P content directly (this of course is not useful for situations where students lack connectivity). I wrote a bit about this for a previous project supporting H5P use in Pressbooks
It would take more effort, but in some cases an alternative activity might be written that would work in print (not quite the interactive experience) that is included in the index… or I have wondering if it might be wrapped in CSS that would hide it when viewed on line but display when printed (to PDF).
So for now there is no “alt” tag for H5P or something that could be generated from the content types itself. I know there is some interest out there, but for now, the only way I can see is creating a different version of the activity that might achieve the same outcome.
Thank you so much for coming in here as a student and asking questions, and please feel welcome to ask (and answer others) anytime.
Keep on cooking with H5P!