Indeed the easyjet H gates can be accessed from the main non schengen departures area. But there is no passport control between H and M so M gates can only be reached from landside. Not really a problem as very few people are doing this type of transit, and those who do have certainly no restriction on entering landside.

On the new A pier for sure there will be passport contol between the schengen and non schengen part. The transit bus is necessary to allow NS to NS transit passengers to avoid entering the schengen zone, but I certainly expect it will be possible for passengers so inclined to make their way from NS A pier to NS D pier without the bus, instead passing through passport control twice. It remains to be seen which route turns out to be faster.