I would never take a dog without meeting them first, even if it belongs to a friend. It's hard to assess real behavior by just going off of what the owner says and sometimes they stretch the truth. You are responsible for their safety and ultimately you have the power to decide if you can safely achieve that even if it means losing money or a customer, it's much safer in the long run.

I would have them seen by a veterinarian ASAP. Ear infections can disturb balance as well as neurological problems only their doctor can diagnose

Expending their energy! A nice long walk or interactive game such as fetch or frisbee will help. Giving them a "job" like hiding treats for them to "hunt". Dogs feed off of your energy so if you are nervous or excited they will be too. If a dog is panicking from loud stimulus (fireworks) there are remedies your vet can reccommend but a good OTC is Adaptil spray or Rescue Remedy.

The general rule of thumb is one hour per month of age for young puppies until potty trained (3 months, no more than 3 hours). Depending on activity level and water consumption, normal adult dogs should not be left crated more than 6-8 hours without relief. For seniors, less time.

Lethargy, lack of appetite, crouching, hiding, whimpering, tucked tail are just some signs. Sometimes, carefully feeling the abdomen and touching lightly if the belly seems tense that can indicate there is a problem.

Dogs natural instinct when wet is to shake off and dry to prevent hypothermia. Running around with the "zoomies" helps shake off the excesss water. Another part of it is as someone mentioned, nervous energy and a release of tension. Most dogs don't want to be held captive and bathed so this is a big sigh of relief and stress release