How does Astral Heron work with "here to help"? Does the discount apply to the unit cost if "Here to help" is played from hidden as the first spell? Does it work if "here to help" is the second spell this turn? Or never? And if not played from hidden, it does reduce the cost of "here to help" by 2 energy, but can the 2 power discount be used for the cost of the unit?

No — Astral Heron's discount can never be spent on the unit Here to Help plays. The discount only ever reduces the single next card you play after the Heron trigger resolves (FAQ #11762); it can't be split or redirected between two cards (FAQ #12624).

The trigger doesn't fire the instant you play your first card — it fires once that card's play is complete, i.e. after it resolves (Rule 419.4.a). So if Here to Help is your first card, the trigger resolves only after Here to Help has finished resolving, and the unit was played during that resolution, before the discount existed. Hidden doesn't change this (the base cost is just ignored, so there's nothing for the discount to reduce anyway) — the discount gets banked for your next card instead.

If Here to Help is your second card — the next card you play after the trigger already resolved — then the discount lands on Here to Help itself. It costs 2 Energy + 1 Power, so you pay 0 and one [A] is wasted (costs can't drop below 0, Rule 356.6). That leftover [A] still can't be applied to the unit; the unit only gets Here to Help's own [3] Energy reduction.

So "never" isn't quite right for Here to Help as a spell, but it is right for the unit: Heron's discount can never reach it.