You wrote, "The crux of the situation for me is whether B
You wrote, "The crux of the situation for me is whether Brigit can be identified as
maiden form of the earth goddess to Cailleach’s hag form."
I see absolutely no reason to draw that conclusion from anywhere in the Irish mythology. For three reasons.
First, because there is not necessarily a "hag form" and a "maiden form" of any earth goddess. Notwithstanding the example you give of the sovereignty figure appearing as an ugly hag till her bridegroom accepts her (which is a late and folkloric theme if memory serves), in the mythology recorded in the early period, i.e. early middle ages, I cannot think of a single goddess who appears alternately in maiden and old woman form.
Second, because if you do accept the late, folkloric "bride turns from a hag into a beautiful maiden" motif as part of Irish mythology around a sovereignty goddess, that goddess is better idenitified as Morrighan, or Macha specifically, or Bua, or some other goddess with tons of sovereignty associations. While all sovereignty deities are female, not all female deities are sovereignty goddesses.
Third, because there is no reason to believe Brighid is an aspect of an "earth goddess" whatsoever. Her mythos is decidedly solar and heavenly. She is the daughter of the Daghda, is associated with the hearth, signifies the beginning of warm weather, has a festival during a time when the earth is not fertile, has a star- or sun-like "bow" (bogha Bride) as her symbol, and is known in the mythology primarily for inventing keening. The only earthly or harvest association I can think of is the use of a corn dolly as a brideo/g which may be a conflation or late version of having a live person dressed in white as the brideo/g which is also an attested practice. Even the saint named after her has strong solar associations in the way she was born and some scenes from her life; both give blessings to protect the house from fire, and nothing to do with harvest or planting. I have a hard time seeing her as an earth goddess.
I really admire what you're doing in trying to understand the Gaelic pantheon but in this case I don't think you have this figure nailed down. I don't know that there are going to be perfect correspondences to the Slavic pantheon but if there are, I would look elsewhere for a harvest goddess. To whatever extent it may help, the clearest classical equivalent of Brighid is Vesta; you're looking for a Demeter.