The 1926 re-registration may have details of her adoptive family (seeing as it appears to be listed as "Taylor") or give some confirmation via birthdate, but as you say it's unlikely we'll get any firm confirmation of the father.
Keightley children can be found in the civil bdms of Paris:
http://archives.paris.fr/r/124/-at-civil-de-paris/6 July 1886 - Anna Josephine (8e)
Act 999 - birth of Anna Josephine Keightley, born the 5th July (registered on the 6th), daughter of Joseph Keightley, 28, cocher (coachman?), and Anne Marie Leroyer, aged 31, domestic servant (married).
9 June 1889 - Marguerite (8e)
act 1063 - same details, only Joseph is now 30, his wife is 34, and her father's nationality is specified as English.
(there may be more I haven't spotted)
11 Feb 1886, 8e, the marriage:
Joseph Keightley, born Myton (Great Britain), 27 January 1859, coachman resident of Paris Rue de Marignan, 18, son of Joseph Keightly and Harriet Cushing, his wife
to
Anne Marie Leroyer, born Saint-Jean-sur-Mayenne (Mayenne), 16th October 1854, a maid resident of Paris, Rue de Messine, 9, daughter of Michel Pierre Leroyer, deceased, and Anne Foucault, his widow, aged 56, no occupation, resident Change (Mayenne) and consenting to the marriage.