Hello Craig,
Do all beaches have 4 floor rails? Yes
So the 3 seater will swap easily with the 2 seater. Yes, but not that easily as the three seater bench is large and heavy. Apparently it will only come out through the side door.
Does the 3 seater, when reclined to the bed position, sit higher than the 2 seater? Yes. It gives you less headroom but a much more practical load area and IMO is the better compromise.
Does the cushion board at the back come in different heights to accommodate this difference? Yes. The Multiflex board is higher in the three seater version and unlike the two seater Multiflex, has a transversal hinged drop down section at the front end. The higher Multiflex deck makes for a much more practical storage space beneath and the hinged section allows for greater flexibility with the seating positions. Two seater bench owners have said that some fridge/freezers and portable toilets don't fit under the two seat Multiflex board. Some have also said that it's neither multi anything nor flexible.
Is it adjustable or set to a specific height? No. Both versions are a fixed height.
Anything else I need to consider when changing from a 2 seater set up to the 3 seater bench? There are only three issues:
1. Storage. Changing to a three seater bench will mean different storage arrangements. You'll lose the large slide out under bench drawer but gain three much smaller under bench compartments each containing a storage tray. You'll also lose the ability to fit the large optional storage box that runs along the N/S of the van and forms part of the bed base on the two seater version.
2. Headroom. Most adults would have sufficient headroom to sit on the two seater bench when in bed mode and with either the roof closed or the upstairs bed in the down position. This is not the case with the three seater bench in bed mode. As the three seater bench in bed mode is higher off the floor, the corresponding headroom is less. Therefore you either have to lie down, sit crouched over or raise the roof and upper bed.
3. Bed Comfort. The two seater bench is basically the same unit as in the SE/Ocean albeit with some minor differences. It provides a very flat sleeping surface and when used in combination with the londitudinal storage box and Multiflex board forms a superb king sized bed. The combination of the three seater bench and it's Multiflex provides the same huge king sized bed. However, because the three seater bench is the same unit as used the Caravelle and designed mainly for passenger carrying it is more contoured for sitting comfort. Therefore when folded down into bed mode it forms an uncomfortable undulating surface which IMO is not that suitable for a bed. VW get round this problem by offering the optional Comfort mattress which mirrors the undulations on the underneath and provides a perfectly flat sleeping surface. Personally I wouldn't want to sleep on either version or the SE/Ocean's downstairs bed without the corresponding version of VW Comfort mattress. These mattresses provide a good flat base surface to sleep on but in all versions the bed is still rather hard for many people's liking. That's why the Zonesleep topper mattresses are so popular. They provide a softer breathable top layer which makes all the difference.
I hope that helps.