rebdoniel,
When poskim “bend over backwards” there still has to be a rational basis in halacha. A posek can’t just make a up something to be matir a mamzer. For Rabbi Goren to have done what he did, at the very least he must have held that a later Bais Din is capable of declaring that a previous Bais Din had not properly performed the conversion and therefore the person was never megayer.
Now it is possible that a later B”D is capable of “passuling” an earlier B”D, but that this power is only used in rare and extreme ciumstances, and Rabbi Goren held that the Langer case was such a circumstance. But at the very least that power must exist.