Thoughts on how to fix a depression in the roof

I would still be tempted to pull the headliner and see what's going on under there, because this could just keep recurring- and one of these times there could be some more permanent damage.
If the support structure under the roof skin is still intact, and just not close enough to support the roof skin; I would go in there with some spray foam insulation and give it a couple of shots between the framework and the roof skin. Not a solid bead, but intermittent shots here and there. It expands so it will support the skin, but won't apply enough pressure to distort the metal or make it bulge. Just make sure you cover everything in your interior, because that stuff is like Anti-Seize- it gets on everything.