Lesbian subtext between Fanny + Mary IS in the novel text

I think this is the right answer. In addition to the kind of soft-focus "lesbianism" surfacing as a result of gender segregation and sublimated same-sex sexuality, there were actual lesbians as well. Given that marriage in the upper classes was still significantly about semi-arranged marriages focused on dynastic and financial advantage and not romantic love, even a lot of heterosexual marriages were about producing an heir and a spare and not much more. So its likely that upper class closeted lesbians would have been more present in society because escaping marriage wasn't really an option socially or financially.

So you probably had both "frustrated" single women with no/limited sexual outlets and women who were lesbians circulating in the same circles operating in a highly gender-segregated world. It wouldn't surprise me at all if that resulted in a certain amount of sexual intimacy, in addition to there probably being secretive and covert networks among actual upper class lesbians, too.

I haven't read the books, but the relationship between Mary and Fanny seems not unlikely to have been one where same-sex relations of some kind would have resulted. A true upper class woman like Mary who was possibly a lesbian would find a woman like Fanny, who lived in but was not of the upper classes and thus had fewer options for both marriage and sexual outlets, an ideal potential lover. You could also argue that Mary's willingness to marry the second-in-line heir who wanted to become a clergyman wasn't just a conniving path to the inheritance, but a desirable least-worst path for a woman who preferred the company of women since it would likely be less sexually demanding because Edward was something of a weaker personality overall.

It was also normal for higher class women to have less fortunate women of higher class as essentially paid companions. Such a situation being taken advantage of as a covert means of carrying on same sex relations by seems likely.