If I Heedless Resurrection a rift herald can I choose the order things will resolve in

No — you don't get to pick the order; Rift Herald's Deathknell actually resolves first.

Because killing the Herald is an additional cost, the Heedless Resurrection spell is already sitting on the chain when the Herald dies, so the Deathknell trigger gets added on top of it (FAQ #6068; Rules 354.1, 357.2). Chain items resolve newest-first, so Deathknell resolves, then Heedless Resurrection resolves afterward.

So the sequence is: Deathknell → play a unit from your hand to your base, ignoring its Energy cost (you still pay its Power). Then Heedless Resurrection → play a unit from your trash that costs no more Energy and no more Power than the Herald (so ≤8 Energy, ≤1 Power), ignoring its cost. Note the Herald itself can't be the target, since targets are locked in before costs are paid.

This is different from kill effects that happen during a resolution (like Baited Hook) — there the Deathknell waits and resolves last. Here the kill is a cost, so it goes on top.