All that holds the handles on are small pins, easily pushed through the handle (after pushing on the plastic escutcheons) with a small punch, very small screwdriver, etc. Spring pressure on the escutcheons keeps them tight to the handle, which in turn keeps the pins from falling out.
As for warping, maintenance of whatever "weather curtain" might be inside the door, along with intact outer weatherstrip seals, usually is enough to keep the panels from warping. I suppose it couldn't hurt to spray the backside of the fiberboard with some clear lacquer or similar, and I've cut polyethylene sheets to the shape of the panel and fitted them between the panel and door, sealing the plastic on the door. These seem to have done a good job on my Herald, despite the fact that my doors do not currently have the outer weatherstrip seals, nor much remaining of the original inner weather curtains!