This addresses a question which was not asked explicitly, but is crucial to the solution of the other questions posed within: how can you know you've identified the correct person named Jane Glasson, and not some other person?
Here are some suggestions for further research, and some ideas for you to think about. First let's start with a brief overview of the research cycle, as outlined in this article from FamilySearch:
- Identify what you know.
- Decide what you want to learn.
- Select records to search.
- Obtain and search the records.
- Evaluate and Use the information.
(after step five, return to step 1 and repeat)
To evaluate the information you have, one useful tool is Elizabeth Shown Mills' Evidence Analysis Process Map (PDF) which she talks about here in her QuickLesson 17: The Evidence Analysis Process Map.
Another important tool is Inferential Genealogy, where we go beyond the information which is on the surface of a single record, and dig deeper, combining information from many records to build a conclusion. In his video class, (handout here), Dr. Tom Jones, CG, CGL, FASG, FUGA warns against "Kinship Acceptance", which he defines as "The uncritical use of kinships determined or accepted by others."
What immediately leaps out at me is that John Short's biography that you found on the Internet Archive is an authored work -- where did the author get the information about John's wife's maiden name? Does he cite his source?
Let's assume for the moment that he was he looking at the same parish record as the one you found. If so, you are accepting his kinship determination that the parish record from Cornwall belongs to the couple he is writing about. What if he is wrong?
In a case like this, I would suggest widening my search, looking in all record groups that I could find that might mention anyone in the couple's immediate family. For England, references like FamilySearch's England Record Selection Table can help; so can the England Wales Checklist published by oneplacestudy.org (archived copy from the Wayback Machine). For a print reference work, Mark D. Herber's book Ancestral Trails describes all the various record types that might exist.
When reviewing records on the US side, you can consult works like the United States Record Selection Table in the FamilySearch Wiki, Kory Meyerink's work Printed Sources, Mills' Evidence Explained (it's not just about citations), and Christina K. Schaefer's The Hidden Half of the Family: A Sourcebook for Women's Genealogy, which outlines the changes in the law in the USA and highlights possible sources where women's names might appear in records.
You're on the right track when you ask about identifying the witnesses to the marriage. Widen the search and investigate anyone -- friends, associates, and neighbors -- who might be connected with the couple. See QuickLesson 11: Identity Problems & the FAN Principle for an example of how Mills identified a woman on a marriage record named Mary Smith (a very common name).
Another thing to try is working up a list of surname variants for Glasson in case records have been mis-indexed or mis-transcribed.
Also, beware of searching too narrowly when it comes to Jane's age. One important thing to consider when doing inferential genealogy is the principle Judy G. Russell talks about in her webinar "How Old Did He Have to Be?" (available by subscription or purchased download from Legacy Family Tree Webinars). By knowing the law, we can make better estimates of someone's age, when the age is not stated explicitly in the records. How old did she have to be, in order to marry in 1821? Is it possible that you've missed a baptism because she was not baptised when she was a baby, but baptised later on? Perhaps it isn't common for this time period, but sixty years later, I have registers with siblings born two years apart who were baptised on the same day.
Re-examine everything you have on the couple -- analyze the information, and question everything, and search more widely. As you think of new questions, write them down so you won't forget to explore them later.