Hi Berta,
I also experienced similar problems during my mock exams. One thing that radically changed my concentration was the time of the day I was doing the exam simulation. As I know I tend to be less energetic early in the morning, and having to reserve a 4-hour slot for the full exam, I usually put the mock exam on my agenda in the afternoon. One day, I had a meeting in the afternoon so I had to anticipate the simulation in the (late) morning. It made such a huge different to me! No tiredness, no sleepiness: I performed much better! Maybe you can play a bit with that - if your schedule allows - and find what suits you best.
As for the other questions, I highly recommend to contact PearsonVUE to check with them - as they are the most entitled to answer them.
I can only share my experience here:
1. I read from the requirements that drinking was not allowed during the exam, so I prepared accordingly, trying to be hydrated enough before starting the exam, and be okay (just) until the break. You may ask Pearson VUE, or even your proctor, if you can't reach out to them before the exam (I know you may experienced longer waiting times than usual).
2. I'd say it may depend on how you do it... one thing I tended to do pretty unconsciously and yet helped my concentration was to read the questions aloud. As this is not allowed in the real exam, I adjusted my preparation accordingly: it wasn't easy, but as you train for it you get used to eventually.
3. I don't think you are allowed to get up even without leaving the screen view... no idea about the breathing though, so I would definitely ask PearsonVUE and/or your proctor at the latest.
Hope this helps, and good luck on your exam.
Gabriella