mounting them on the metal will be fine. just find a place where you can stick or bolt them on where they won't get wet and they won't get in the way with the door panel. if you have trouble doing that then look inside your kick panels, there should be a place there for them.
That looks fine, as long as you can get your door panel on then you'll be ok. no need to wrap it with anything. how did you get it to stick to the door? bolt? double sided tape?
it has bolt holes for a reason. you could cushion the back sid of the xover with foam or carpet or something.
How about that steel brace, you can zip tie it to that and just add some silicone on the back side to keep it from vibrating. But where you have it is good also.
That metal piece that is like 12 inches long and 1 inch wide, in the center of the door. If you zip tied it very tight with silicone on the back side might be a good place if there is enough room. Just a suggestion.