So poor Mary was insane in 1848. If that's the case, it may be unlikely she remarried or left the country, but it's possible she ended her days in an asylum somewhere, which may not necessarily have been Mylor/Falmouth or Cornwall - unless she made a recovery at some point.
Have you seen the bp for Mary Rowe 23.10.1785 Mylor to Arthur and Susanna? This couple also bp Elizabeth 1808 Mylor (possible Eliza in the 1841 census?) and a son Henry bp 1797 Mylor.
Arthur Rowe looks to have died 1825 Mylor, age 67 years. Possible burial for Susanna Rowe 1845 Mylor age 83 years. Susanna is in Mylor in the 1841 census with married daughter Susan Sara, and it looks as though Arthur and Susanna's daughter Mary also may have married a Sara - George Sara 1815 Mylor. In 1841, George and Mary Sara are in Mylor with a son called Arthur - which may put this lady out of the running for Mary McDonald.
There is another Rowe couple in Mylor - John and Mary - who seem to have bps for three Elizas (1794, 1796, 1798 - last one Eliza Louisa). They also have Richard Miners Rowe bp 1803 Mylor and it’s interesting that Thomas Rowe married Joanna Miners in 1806 Ruan Lanihorne. They also bp Frederick Hammond Rowe in 1807 Mylor, and Frederick Hammond (not Rowe) married Ann/Nancy Blee 1812 in Mylor - Nancy Blee was a witness on the McDonald-Rowe marriage entry.
There are several possible marriages for this couple although a likely option is 13.10.1794 Mylor - John Rowe of Lanivet to Mary Rowe, spinster. The first child, Eliza, was bp 28.12.1794 Mylor and so this was either cutting it fine, or the child was already born.
I can't find a daughter Mary for this couple but wonder if she may have been born before they were married, either illegitimate to mother Mary, or perhaps mother Mary wasn't a spinster when she married John Rowe, but had been married previously. Requires more digging.
EDIT: There’s a marriage for (yet another) John Rowe to Mary Minors 1783 Kenwyn, bride of Mylor, married by licence. Possible children (all bp Kenwyn) Thomas 1785, Mary 1788, Joannah 1789.