Can an ambush unit be played to a valid battlefield if being played from Rift Herald's deathknell effect?

No. Rift Herald's Deathknell says "Play a unit from your hand to your base, ignoring its Energy cost" — the destination is fixed, and Ambush can't override it.

Ambush only adds an option to play to a battlefield where you control units (Rule 822.1.b); it doesn't change where an effect like Deathknell tells you to play. The unit is played by following the Deathknell's instructions, so it must enter at your base. Ambush's [Reaction] timing also doesn't apply here — you're following the trigger's resolution, not playing the unit at Reaction timing (FAQ #11070).