How Does Elnuk Herd and Rockfall Path interact. Can I not flip it from hidden to get the +3 effect @ Rockfall path?

No — you can hide Elnuk Herd at Rockfall Path, but you can never flip it there, so the +3 never happens.

Rockfall Path's "Units can't be played here" beats the Hidden permission ("can't beats can"), and a hidden permanent must be played to the very battlefield it was hidden at (Rule 811.1.d.1). So attempting the reveal is an illegal action: it's rewound, the card stays facedown, and it just gets trashed when you lose control of the battlefield (FAQ #6500). No unit is played, so the "when you play me" trigger never fires — no +3, and no recycle either.

The one way around it is an effect that changes where you're allowed to play the unit (e.g. "the next unit you play this turn can be played to any battlefield"). Then Elnuk Herd gets played to that other battlefield instead of Rockfall Path, the +3 goes on a friendly unit at that battlefield, and it recycles as normal.