] were Catholic marriage ceremonies somehow less legal than Anglican ones at the time?
IIRC, it was in "And the Bride Wore" by Ann Monserrat that I've read that before Catholic Emancipation Act (1830s) even Catholics had to be married by an Anglican priest for the marriage to be legal; and the couples of Catholic religion usually went through two ceremonies - one Catholic (more or less secret), another Anglican.
] which just makes me appreciate JA's subtlety and realism all the more.
Makes all of us, in fact! Strictly speaking, Olivia wasn't to blame, as she went through all of it in good faith. But she blames herself so hugely that I, for one, see her reaction as just another minor Clarissa (who died so long and so heart-brokenly; and then Richardson's plot had many imitators). A little aside: I remember reading Emma, where both Robert Martin in VoW and Harriet in Romance of the Forest (which she wanted him to read), actually met the same or almost the same bad nobleman abducting young ladies on grand scale. A cliche of contemporary literature, if there ever was one.