There does not seem to be any credible evidence [& no contemporary evidence at all] that Charles Marsack was anything other than the son of Charles Marsack & his wife Margaret Saunders. As far as I can discover he never made any such claim & certainly married and died as just Charles Marsack. It doesn't even make sense for his mother to supposedly "Marguerite Countess of Marsac" when "Marsac/Marsack" was in fact the name of his father.
Apparently, the stories of the relationship with Frederick, Prince of Wales [who did in fact have plenty of illegitimate children and freely acknowledged them but did not count Charles or his supposed sister Mary among them] was in fact a later invention by Charles's half sister Margaret Houlcroft who of course had no claim to Marsack as a surname as she was a product of Margaret's second marriage to John Houlcroft/Houlcraft.
There is a Wikipedia page on the subject Geni Major Charles Marsack
But the basic facts are as follows.
Jean "Charles" Marsack, son of Charles Marsack & his wife Janne/Jane Sequain married on the 10th Feb 1746 at the French Chapel, St James Palace to Margaret Saunders, daughter of Edward & Margaret Saunders of Tunbridge, Kent.
He died in 1571, leaving a widow & son.
In 1753, Joachim Lorenz Sollicoffer/Solicofer wrote his will, leaving 300 pounds to Margaret, along with a gold watch & plate, and 200 pounds to her son Charles, his Godson.
Charles Marsack made a career in India, before returning to England in 1783, when he married Charlotte Becher, daughter of Richard Becher [who had been a director of the East India Company. Charles & Charlotte had 11 children before he died in 1830 & she died in 1837. He made no claims of any Royal ancestry that I can see.
Surely his father should be Charles Marsack rather than the Prince of Wales as there appears to be no evidence at all for the latter?
My connection is via the Becher family.
From Jenny Joyce [now Stiles. Daughter of Pamela Becher]