Whilst you’ve attempted to lay out some of the context in the question, I think there are a few reasons why it is hard to give a complete answer.
The context is incomplete:
- What is the relevance of the fence, does it stop the non-speakers from getting near the stage, how high is it, it solid or does it allow light through?
- It is 10 AM, what does that mean for light in the location of the protest, are the crowd gathered such that the sun will be behind the dais, behind the crowd, to the side?
- What are the limitations on crowd position, are they all in front of the dais, are some behind it?
- Is the area open prior to the protest, so that you can find a location, or do you have to move into it with the masses?
But for me, the biggest missing piece of context is what is it you are actually trying to achieve from the photographs? What is it you want to show? Is it the emotion on the faces of the speakers, the feeling in the crowd, a mass of bodies, tension on the faces of any authorities present, or something else? All of these will influence the best place to stand and where to focus your attention.
Taking your last question first, ‘Is it acceptable to get pictures of the back of heads?’, Yes absolutely and No it’s not. It’s very subjective and entirely a personal decision depending on what you’re trying to convey. Holding the camera above your head to take a photo of the dais, showing the tops of thousands of heads between you and the dais may be the photo you are after, or it might be very disappointing depending on your perspective.
As to your other questions:
(1) It depends on what you’re trying to capture and the environment...
- If there are buildings behind the dais, you could shoot pictures looking down on the crowd, as if from the dais to emphasize the power of the speakers over the crowd.
- If you are close to the front of the dais, taking pictures from as low as possible, up at the dais may give the impression of the speakers towering over you.
- If you can gain sufficient height to get the POV at eye level (shooting from far back, or from a building, or camera held above your head, or on somebody’s shoulders, or camera mounted on a tripod held above your head and activated with a remote shutter release or some combination) with the speakers you may be able to give the impression of being on a par with them...
- Shooting from behind at least some of the crowd may allow you to demonstrate how the speaker has captured their attention.
(2) Personally, I’d try to keep the subject of the photo in focus (be it the speaker, an individual within the crowd, the front of the crowd etc)... Beyond that it seems like an artistic decision as to whether or not your want other elements of the picture focussed or blurred. Are you attempting to express individualism within the crowd, or a homogenous mass?
(3) I generally don’t follow the rule of thirds...
(4) Again, this seems somewhat artistic... If you’re shooting long distance and you want sharp pictures, you’re going to need a fast enough shutter speed (obviously available light and its location may impact the viability of this). If you’re looking to convey being jostled in the crowd, then you may be happy to have some motion blur as part of your photos...
My suggestion would be to think about what it is you’re trying to achieve with the pictures, read the “how to protect yourself” question linked to by @ahockley in his comment and then if you need more help, ask more targeted questions that help you on the way. So you may end up with something like:
- How can I isolate a single face in within a crowd when I have little ability to move?
- How can I convey a feeling of ‘being insignificant / power / XXXX’ when shooting a speech from within a crowd?