I thought about using a lower gray panel like you're describing on my black panel to cover the wholes in the speaker cover that the previous owner made to make room for speakers. I haven't gotten around to trying it yet though.

I don't know how you could patch the crack on the front, but you might be able to keep it from getting worse by taking a plastic welder and attaching some extra plastic behind it to keep it from flexing at the crack. Be careful if you try this, the plastic on the panel is really thin, so you could burn through. I used one to reattach the handle on my chopsaw when it snapped while I was using it last winter. Worked for a couple months before the pressure from pushing down on the handle ripped the patch off. A door panel wouldn't really have any pressure on it though.