Your points are valid, and I am and was aware of the "gassing issue" The expanding foam seems to have prevented any flexing or gassing issues. The body filler once it was sanded away was VERY, VERY minimal. Also the the bed liner is a high build element and it hides the exposed foam very well. And last is It's been in the sun for three days now with a wind sheild on top of it and I've had no problems yet. Not to mention this is not what I beleive to be a long term cure but a nice solution until one can afford a new, correct dash.
I'm with you. I think using the fiberglass can make it a long term fix, I'll be posting pictures of two differnet dashes, one is a 1750 Berlina where I used fibreglass then painted with "Hammered Metal Paint" Between the paint and sanding marks it looks good, similar but not exactly like the original. The 2000 berlina used fiberglass with bedliner, it really looks original. The first dash I did took three tries, the bubbling/lines seem to happen after about two weeks after being in the hot Socal sun. The first one I used SEM vinyl paint, which usually works great, but after being in the sun, it did wrinkle on the dash over the bedliner. I also think thinner rather than thicker for bedliner is preferable.