I would doubt it being from inside the door, the plastic is only a vapour barrier really. Even without it, the water should run down inside the door glass and drop from the drain holes at the bottom to the outside. It is worth checking the door rubber seal all around, including the top - the water can wick around the seal. Peel the rubbers back on the inside and check for any wet or staining.
In my experience, most wet front footwells are from water coming down the a-pillar or bulkhead from a leak above. They can be pigs to find, you will probably have thought of most of the usual suspects - blocked scuttle drains, windscreen bond, aerial, sunroofs, bulkhead grommets for wiring or heater pipes, etc.. Take the wheelarch liner out and look there and underneath the footwell for any suspect areas, I have found body seam sealer split before, or even missing completely.
The reason the wet is at the bottom is just because that's the point where it collects.The leaks are often so small that they trickle down metalwork in beads and show no trace elsewhere.
You might have to resort to getting the carpets up and and an assistant to apply a hosepipe. Remember, you will only be looking for the odd bead of water, not a gusher.