Other relevant deities connected to Leto:
Most snake cults have associations as guardians..commonly over wells, rivers etc. Although Leto’s cult is not overtly a serpent cult (and other than her syncretism in Egypt we typically don’t see serpents directly represented with Leto) it should be given that this extensive divine family is a strong serpent cult even without the direct markers for Asteria or Leto both. It is then of little surprise that those deities connected most overtly to her are river gods. Peneus who offers her refuge from Hera suggests a cult association that is quite probably lost to time whereas the relationship between her son and Peneus continues to be a strong one, just as we tend to think too when it comes to the god Xanthus of the Scarmander river. Yet in the latter example, although more mythically connected to Apollon, it is a temple of Leto that was raised near the river. It is quite possible that the myth of the Lycian frogs deals conceptually of the relationship between water guardian serpent cult element and that the rejection and driving off of the goddess by the populace was addressed as a personal afront to to the nature of this goddess who may have made her blessed guardianship there and whose raised anger at being driven from the river not unseeming when taken with that in mind that the people themselves became the very spirit of the water they covetted as frogs rather thsn benefitting from her presence at the edge of the waters. Certainly we see a similar role with Artemis Eurynome who although bears cliser resemblance to a mermaid still by her function and form directly inherits this cultic nature as protecting goddess if the river where Rhea bathed Zeus. It is quite possible that Leto may have originally had at one time just as many rivers associated with her as Artemis does.
It is perhaps in this light that we can understand the relationship between this family and Zeus (given his cult forms as Kteosis and Melikhios with its strong serpent form and symbolism). Zeus, who was fostered by Ge, has a strong chronic nature as a god who has taken residence at times within the walls of the earth and exposed to all if its hidden mysteries and the serpentine quasi chthonic cult of this divine dragon family and there connection to rivers slipping in and out of the earth carrying forth hidden secrets (which is why many oracles are attached to oracular waters) and relevatory prophetic and divinatory nature. Certainly the level of respect he pays to Hekate demonstrates this, as well as his failed attempting at bride catching Asteria. Yet it is Leto who is listed as an early bride of Zeus and by her womb transitions the authority of prophecy from the night and earth to the rule of the aether of Zeus’ s domain by his son Apollon, while not the only deity to possess oracles his is the official office which cannot be slotted elsewhere as Apollon explains to Hermes. There seems to be here a very clear lineage relationship at play here. Even more so when Apollon is noted as heir of Koios.
I would wager strongly that Hermes, given his interaction with Leto in the Iliad also as a strong connection to Leto that us no longer explicitly clear. My opinion is that whereas people identify the serpents of his cadeceus with agathos daimon and agathos tychke as being respectively the symbol forms of blessings of Zeus and Hera, that given what I gace said above it may be plausible to be connected also to a “marriage”of the blessings of Leto and Zeus.