how can 2 EM waves (travelling in opposite directions) null each other at a point in space but continue to propagate beyond the point in space where they interact to null each other?
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You describe an impossible scenario.
If two electromagnetic waves travel in opposite directions and their electric fields point in opposite directions, then their magnetic fields point in the same direction.
If two electromagnetic waves travel in opposite directions and their magnetic fields point in opposite directions, then their electric fields point in the same direction.
More generically, when the electric and magnetic fields both pick up a minus sign, the Poynting vector remains the same. So the momentum density can't point in opposite directions when the electric and magnetic parts of the electromagnetic field are both opposite.
If you want to get super technical: a zero field can satisfy all these things. But then there are no waves and no energy and no dynamics.
Timaeus
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