Your argument regarding the three reasons why la viuda led a lonely and sheltered life goes hand in hand with the story "La Viuda de Montiel", especially towards the paragraph that reads:

Aquella mujer frágil, lacerada por la superstición, casada a los 20 años por voluntad de sus padres con el único pretendiente que le permitieron ver a menos de 10 metros de distancia, no había estado nunca en contacto directo con la realidad.

There are basically your three main reasons: first, in typical García Márquez style, women are either too dependent or too independent. Very seldom do we see a "normal" female prototype. Therefore, the first reason is the submissiveness of certain types of Hispanic women, who still abide by the dictum that "man" is he head of the household and that women are meant to serve and obey them.

...casada a los 20 años por voluntad de sus padres con el único pretendiente que le permitieron ver a menos de 10 metros de distancia

The second reason, is because such submissiveness is also a part of the Hispanic ideal that a virtuous woman moves from the house of her father to the home of her husband. Little to no importance is given to the opinions, wants, or needs of women because the assumption is that the job of the woman is to produce offspring. The women seem to understand such notion and, as such, they acquire the behaviors and attitudes that are inherent to a servant, or even a slave. This is purely consensual, however, and within the historical context of the story, it should not be seen as abuse except when looked through the modern eye.

Third, the story is clear that she had these not-very-redeeming qualities: she is fragile, superstitious, and out of touch with reality "as it is".

 Aquella mujer frágil, lacerada por la superstición...no había estado nunca en contacto directo con la realidad.

Therefore, what we have is a woman who may already possess an eccentric and strange notion of the world around her and, as it is expected of her cultural background, she is given away in marriage to a man who can support her. We will never know as readers whether her superstition, fragility of body, and unrealistic view of the world are natural or a result of being a woman whose needs have never been met. However, we can certainly correlate this type of behavior with the choice that she makes of basically burying herself within after the death of her husband.