1) The Calibed mattresses (I have the older Roof II, for the T6) for me are much better than factory. They provide more support (I no longer feel the slats in the T6 bed), while at the same time they are softer and more ‘anschmiegsam’, which I cannot translate even to Dutch. Also warm in winter, breathable in summer. The trick seems to be in the two layers of high quality foam, combined with an extremely breathable, supple and elastic outer fabric. Other suppliers may achieve the same, although I haven’t seen anyone else do iterative improvements quite like the Calibed series. The Vanessa Mattress only seems to replace the inner foam by a two-layer foam, using the VW outer fabric.
The T6.1 bed should be more comfortable already with the dished design, so I would only upgrade after trying the original for a few months. For me any mattress has to store in-place, with no faffing about, so no toppers and inflating mattresses.
4) I have the downstairs mattress and duvets on top of the rear shelf, and throw my coats on there. If you don’t sleep downstairs, you could store lots there. Maybe something like the Thule packing boxes would work well for organising the space? In the boot I use euro boxes.
As for the sliding boot tray: just start without, and if you miss it you will know.
5) I now have HEKO wind deflectors. Not happy: they are a shade too dark for my liking (blocking the view out) and one of them fell out during the heat wave this summer. Before I had some other brand that fitted with a small screw instead of just double-sided sticky tape. That worked better, they also had a lighter colour. I will try to order those again, once I remember the brand name ;-)
* For the wardrobe shelves I prefer these:
https://calitop.eu/product-13-14.html?language=en . I prefer to keep some space in front of the shelves for hanging fancy clothes if needed.