] Am I correct in understanding this? Cassandra Leigh's mother was the one with the impeccable ancestry (the Perrot side of the family) and some money to be inherited, but who married a clergyman (not so high in social status). Cassandra Leigh, in turn, with good ancestry on her mother's side married a clergyman (like her mother), who had a very modest living. So two generations of women married into a lower social strata.

Not exactly, since both sides of Mrs. Austen's could be considered aristocratic. Mrs Austen's mother was née Miss Jane Walker, so yes, that is the Perrot's side since Mrs. Walker was born Miss Jane Perrot and married another clergyman, Dr. Walker.

But also the Leighs had ancestry, from her father's side, Mrs. Austen descended from a Mayor of London (Sir Thomas Leigh). Also her father, Rev. Thomas Leigh was the son of Mary Brydges, sister of the 1st Duke of Chandos.

But I have not all the Perrot's ancestry, am I to understand that they were only landed genty, but not titled gentry as the Leighs?

So all in all, Mrs. Leigh, Mrs. Austen's mother, did not married beneath her, but equally IMHO.