"Used by G. M. Hopkins: probably a blend of twist v. and dwindle v. Apparently an isolated use." OED

At this time, charities would publish "subscription lists" with the names of all the wealthy elites that donated to their cause. These annual reports listed the names of each annual subscriber and the amount that they gave that year, so the wealthy could show their generosity, and charities could increase donations by making it appear fashionable to donate (this is why Mr. Parker wants Lady Denham to subscribe). http://eprints.lse.ac.uk/61080/1/Flew_Unveiling_the_Anonymous_Philanthropist.pdf

The value of a guinea was equal to 21 shillings at this time (the value was fixed at 21 shillings between 1717 and 1816), but the coin was being phased out by 1813 and was taken out of circulation in 1816. Consequently, "guinea" was became a colloquial term for a unit of currency. https://en.wikipedia.org/wiki/Guinea_(coin)#Eighteenth_century

"Subscription" meant a legal promise to pay a certain amount of money, and was a common form of charitable donation. http://www.oed.com/view/Entry/192959?redirectedFrom=subscription#eid

https://histphil.org/2018/08/13/expressive-anonymity-what-pseudonyms-in-19th-century-charity-subscription-lists-tell-us-about-donors/

It could be considered scandalous for two young, single people of the opposite sex to be having a completely private conversation. Many couples were alone together for the first time during a marriage proposal. https://byuprideandprejudice.wordpress.com/2014/01/28/courtship-and-marriage-in-the-regency-period/