I don't think the timeline fits
Richard was born April 1791 so was conceived around August 1790 - Ann was on Norfolk Island from 8 August 1790 (left Sydney 1 August 1790)
Simon Lavender arrived on Norfolk Island in November 1791 - 7 months after Richard was born and after Ann had married James Morrisby
I think Leanne has it correct: it is unlikely that Simon Lavender was Richard's father; it is also unlikely that Ann was ever known as Lavender [see notes 20 and 25 in my referenced story]. Ann and Simon were in Port Jackson at the same time for a month in 1790 but it is unclear why the name Lavender was ever associated with her, or with Richard, or with other of Ann's children, though there is similar uncertainty about the name Larsom (carried by Richard's descendants) however. As to Ann's antecedents, DNA evidence currently suggests that her mother was named Ann but no father has yet been suggested by DNA associations.