the reason that water leaks into the footwell is not readily apparent but if you pull the rubber door seal off after a rain shower or washing the van you'll see where it is coming from
The video I made some years ago shows exactly where the water gets into the footwell . (see link below)
The A post channel is an excellent method of preventing rainwater access via the door seal rubber .If you do not wish to fit a A post channel then another good solution when replacing the door seal rubber strip it would help to fill the channel of the seal with silicon which will prevent the water getting behind the rubber seal
Water can also get into the footwell from the windscreen to scuttle juncture . Once the floor carpet and trims have been removed you can follow the rust and water stain marks to see where it is gaining access..If the water in the footwell is inboard at the back of the footwell then it is coming in along the scuttle not the door seal
Another place where water can get into the footwell is via the windscreen rubber seal adjacent to the roof gutter. If you peel back the windscreen seal next to the gutter you'll see where water can run down behind the windscreen seal between the rubber seal and A post and exit under the dashboard onto the floor near the accelerator pedal where it then runs into the footwell (see last part of video )
https://youtu.be/d3X2zlsvMbE