Had a look at the will today (LDS film 0106406) and it appears to be as you say.
As this was a substantial estate - trust funds, bequests of 㰰 - it is very likely that there are other documents relating to it.

Do you have a death/burial date for Mary Kayll?

The will is headed Douglas, but a quick look does not show a burial at Braddan, St George, or Onchan around 1799.