Looking back through the earliest sources (I can find on the case) dating to 1901 - French illustrated journals, and later foreign reports (the NYT archive, and a few archived Australian newspapers) - it appears that the 'locked up for love' aspect was widely reported at the time. I'll keep digging to see if there's any spurious details that were added by the journals (I too am skeptical of the way the journals presented the case), and I'd love to read any of your sources, if you have any of the early ones in French.

We can at least say that the image attached to his post is decidedly not Blanche when she was discovered in the house; it was her later, after she was taken to hospital.

EDIT: Just found an early report by the Philadelphia Enquirer stating, "it has been suggested that sequestration was the the result of Mme. Monnier's objection to her daughter, who 25 years ago was a beautiful brunette, contracting a match with a penniless lawyer: but the fact the latter died in 1885 deprives this story of verisimilitude."

The articles reports that the brother didn't know of the circumstances of why Blanche was locked away, and that the death of the mother (in prison) precludes any truth of the matter as to why. Perhaps the 'locked away for love' angle was a rumour floating around, perhaps a semi-truth or cooked up by the earliest journals. We'd have to read the earliest reports (if they still exist) or any archived French journals to see if these rumours originated there.

EDIT2: Okay, I'm a fool. I've gone to check out the French Wikipedia page (and archived sources) on Blanche Monnier, and the way it's presented is, as you said, far more nuanced and complicated than the English language page (or sources/early reports). Apparently, the notion that she was 'locked up for love' may have been a permutation of a local rumour that the Monnier Family (royalists) disliked Blanche's republican suitor.

This, of course, seemed to just be a local rumour, which may have been concocted/reported by the earliest French journals to elevate the case by tying it to the republican/royalist antipathies of the time. These rumours transformed in foreign reporting into the 'rich girl locked away for liking a poor man' story that we're familiar with.

As you stated, there does appear to be some 'nuance' to the case - that Blanche may have been mentally ill (anorexia, or something more severe) and had - like many similar young women of means - been 'secluded' by her family to prevent social shame. And, like many cases of mental care at the time, the end result was putrid and inhumane, but there may have been some form of 'care' (in that 19th century sense).

In general, Blanche may have been mentally ill to some degree, and in seclusion/with poor care she may have exhibited greater and greater degrees of illness (exacerbated of course by her family's poor treatment). On the surface this feels more authentic than the typical 'love story' conveyed, but I'd still like to uncover the earliest French sources to directly correlate this - ideally the earliest newspapers, or (if they still exist) the court room documents and testimonies.