OK now.
After a weekend researching products and procedures, I think I have formed a view, but I would still be interested in anyone's views (and especially yours, Meener Dinkla)
I am going to work with the official Dibond product (the brushed silver version) and aluminum U brackets. I was almost sold on the expanded PVC and e-panel solution, and I did locate local vendors of these products on my island. I became convinced that with the right adhesive between the PVC and e-panels to manage their different thermal coefficients of expansion (some specialized polyurethane adhesive as mentioned by Ernst and additionally his use of the vacuum press), I think that is an elegant solution for anyone and should be perfectly fine. Unless -- and here's my pivoting point -- one must ship most of one's product several thousand miles through multiple vehicles and modes of transport for delivery to customers. I am pretty certain in my case that the weight of the PVC panel would end up causing unwanted damage as the package is tossed around in transit -- especially from my location. If my market were more local, I would have pursued that option.
By acclamation in my research, Dibond appears stronger and will by itself remain flatter due to its thicker and stiffer aluminum exterior. It is actually used to build furniture and in construction. E-panels may suffice, but Dibond should do better when it must exist without the added support that would be provided by a PVC board backing. Using aluminum brackets saves a lot of weight and makes shipping through the carriers less problematic. Also, as a secondary consideration, I don't need to worry so much about separation of the Dibond and aluminum brackets due to different thermal expansion rates since they are nearly the same.
I was very tempted and was truly in the process of placing material orders to follow your method, Ernst, until i thought about additional shipping damage and costs. I can't exaggerate about how many damaged articles I receive in inward bound shipping where I live. That really is the deciding factor for me. Does that make sense?
By the way, in case you've never been here, here's my favorite subject, Haleakala. Haleakala ("House of the Sun" in the Hawaiian language) is a mountain over 3000 meters high. It occasionally snows.