5k requires nothing more than some water after finishing. I do not need to take water during a run until 10k+ but will generally carry water over 8k+ just for comfort. Sports drink, gels or recovery drinks are unnecesary until around 20k distance.
I did something similar training for mine. Assuming the long runs are at a steady pace the missing 3 mile is not going to ruin your training. I would worry more about the following week and how running a 10k race on sunday will then affect what you have planned for mon/tue.
In terms of PB prep it depends on your priority. Personally I would worry more about keeping the HM plan than clearing the path for PB. I have had a few 5 or 10k PB's "accedentally" come during HM training anyway.
Depends. Is it your first? Have you trained well? Are you experienced or novice?
You will basically have a bad day. If you are starting that bad day from a high level then no biggie if not then better to be safe than sorry. Especially if the bug is preventing hydration which it probably is.