Lady Denham obviously favored her second husband over her first, even though Mr. Hollis left her with an inheritance, which is much more useful than a title.

Pales are the stakes of a fence that represent a property boundary.

https://www.merriam-webster.com/dictionary/pale

This is around the same age as some of Austen's male love interests. Mr. Darcy was 28. Many of the male targets of Austen's marriage plots were older than their female counterparts.

(adj) present in excess of the normal or requisite number

http://www.oed.com/view/Entry/194438?redirectedFrom=supernumerary#eid

It is unusual that these young girls were able to spend beyond their means, because at that time women's finances were always controlled by men.

The Parkers were hoping for a much larger group in order to bolster Sanditon's economy. The fact that they arrive in a hack chaise is also a sign that they aren't as wealthy as the Parker's had hoped. A carriage is the superior method of transportation. Mrs. Bennet comments in Pride and Prejudice that Mr. Darcy refused to talk to Mrs. Long at the ball because she arrived in a hack chaise.

Jane Austen's Vehicular Means of Motion, Exchange and Transmission:

https://www.erudit.org/en/journals/lumen/2004-v23-lumen0265/1012194ar.pdf

"Bloodletting procedures, including leeching, became the most common medical procedure throughout the early modern period. By the early 19th century, many patients regularly submitted to various bloodletting practices as a means of preventing or treating infection and disease"

https://www.britannica.com/science/leeching

Leeching more specifically in Jane Austen's world: "One can imagine that during her final illness, Jane Austen was no stranger to leeches."

https://janeaustensworld.wordpress.com/tag/leeches/

"Wagons, called Bathing Machines, were invented especially for the purpose, and would be drawn out into the water by sturdy women, who might then assist you down into the water where you could paddle about or swim in relative privacy, shielded from view of the shore."

https://www.janeausten.co.uk/tag/bathing-machines/

Dipping in the sea was said to have healing properties.

"Jane Austen’s Sanditon, Doctors, and the Rise of Seabathing" http://jasna.org/publications/persuasions-online/volume-38-no-2/darcy/

These are not established charities. Diana is simply naming families that have faced hardship.

This is a mischaracterization of Lady Denham because she is anxious about her finances. She is worried about her existing investment in Sanditon, and she believes that her heir needs find a wealthy spouse.

Mr. Parker is asking his wife to solicit Lady Denham for charitable donations.

Commentary on women's education at the time. Women were considered accomplished if they excelled at the arts and music, but these skills were not useful. Without a proper education about history etc they remain ignorant about the world.

Austen uses two seemingly opposing words to describe Miss Lambe. Her disposition is chilly, but people are willing to over look that because of her money.

This comment is ironic because Diana makes of a point of convincing herself and others that she is ill when she isn't.