when you fail to conquer as attacker, do the ready units go back to their original place exhausted?

No — a failed conquer doesn't change a unit's ready/exhausted state. The attacker is recalled to Base, and Recalls don't exhaust or ready anything; each unit returns in the state it was already in (Rule 453.1).

So a unit that exhausted to make the standard move will come back exhausted, but one that was ready when it attacked (say it was readied by an effect like Ride the Wind) returns ready. Recall also isn't a Move, so no move triggers fire from it.