From as far back as ancient Egypt, the health benefits of donkey's milk has been recognized. This milk supposedly is "anti-inflammatory and hypoallergenic" and has vitamins and probiotics that make it a more nutritious drink than other animal milk.

https://foodtravelist.com/donkey-milk-health-benefits/

Many cultures believed that the nutrients from the milk aided in the prevention of and recovery from diseases.

https://www.sciencedirect.com/science/article/pii/B9780128097625000310

Physic is a dated term that, in this context, refers broadly to "medicinal drugs."

https://en.oxforddictionaries.com/definition/physic

Consumption is another name for tuberculosis (TB), which is a serious bacterial disease of the lungs. In the 19th century, there were no cures or effective treatments for consumption, making it one of the leading causes of death and a serious fear.

https://www.lung.org/about-us/blog/2016/01/how-we-conquered-consumption.html

It is possible Susan was suffering from a headache caused by clenching her teeth or from having sensitive jaw or neck muscles. It is not, and has never been, a legitimate cure to extract teeth as a result. This extreme measure highlights that the Parkers are hypochondriacs.

https://migrainepal.com/treatments-clenching-grinding-headache-and-migraine/

Cross friction massage therapy is an actual medical technique for sprained ankles where one applies pressure to the injured ankle using one's hand. This breaks down scar tissue, which would prolong the healing process. Usually, this technique is done a few days after the injury, not immediately as Diana suggests.

https://collegeofmassage.com/toronto/2013/08/sprained-ankles-and-massage-therapy/

Bile is a bodily fluid that is produced by the liver to help the small intestine digest food. It is possible that Diana is suffering from a condition called Sphincter of Oddi Dysfunction (SOD) where bile gets intermittently backed up between the liver and small intestine resulting in severe and seemingly random abdominal pains.

https://www.urmc.rochester.edu/encyclopedia/content.aspx?ContentTypeID=134&ContentID=181

Austen is most likely drawing upon her own grievances, as she often complained of "bilious attacks."

https://mh.bmj.com/content/31/1/3

Hypochondria was recognized as a real condition during Austen's time. It was classified as a "nervous disorder," and tended to be reserved only to the elite of society. This seems to be the only illness of Susan, Diana, and Arthur.

http://jasna.org/publications/persuasions-online/volume-38-no-2/darcy/

In 1753, Dr. Richard Russell popularized the idea that sea air was beneficial to one's health and nervous system in his writing A Dissertation Concerning the Use of Sea-Water in Diseases of the Glands. By the 19th century, it was generally accepted that the air, in addition to the actual water, had health benefits, but this belief was not backed by science.

http://jasna.org/publications/persuasions-online/volume-38-no-2/darcy/