It sounds like a broken truss to me as well. I am not sure if the metal you have would work or not. If it is just flat metal, probably not. It is probably possible to use the metal to patch with, but it is unlikely to span a ponding area without the metal eventually depressing down into that area. Just adding foam to support the new metal may not work either if indeed, the truss or trusses are broken or have come apart. In roofing, 'Ponding' is standing water that remains 48 hours after a rain storm. To be considered ponding, the water would be 24" in diameter and 1/2" deep or 36" in diameter and 1/4" deep, or larger.
Assuming that you do have a broken truss or two, the trusses should be fixed. Short of that, the sealant used in the ponding area would have to be a product that is proven to stand up to ponding.
The only two products I know of (I am sure there are many more) that advertises it's ability to withstand ponding water. One product is Liquid Roof (better than Liquid Rubber by the same manufacturer). http://www.epdmcoatings.com/
The other product is Elasto-barrier, by Ame's Research http://www.amesresearch.com/
. The liquid roof product is much more expensive, but it (should) goes on in one coating. The Elasto-barrier would need a minimum of 4 coats, about the thickness of a quarter when fully cured. The Elasto-barrier is also a medium gray color and should be top coated for uv protection. I use Ame's Maximum Stretch. Between the two products, I use the Ames products much more often. I am not aware of any 100% acyrlic product (Kool Seal, Snow Roof, Snow Seal, Henry's Elastomeric, etc.) that can survive ponding. It breaks down, wrinkles up and peels off.
I would not use black asphalt type sealants (Henry's 208) on a metal roof, except for a small emergency patch (Wet/Dry), which would be later removed and replaced with an elastomeric.
Repairing trusses is an advanced repair, but two able-bodied, construction capable people could do it. The repair could be done from the roof or from inside the home. Working from the roof means you are installing a new roof, because you will destroy the old roof. From the inside, first run a razor knife along the joint of the ceiling and wall. I keep a slight outward cant on the cut. Then remove the plastic strips (splines) to expose the staples holding the ceiling panels up. The splines will undoubtedly break some due to drying out. You'd have to replace these with painted wood strips or something of that nature. With the staples exposed, cut the middle of the staple with wire cutters and pull each side of the staple out and repeat about 100 times. Keep some pressure on the panel as without the staples it will want to fall and break. I usually leave 2 staples in on each end, about 1/3 the way in from the end. Then two people get on small step ladders, pressing their head against the panel while removing the final 2 staples.
At this point, you might see a clear plastic vapor barrier with either batt or blown insulation. Blown is a major mess. But pull that stuff out to expose the trusses. I make repair patch sections out of 3/8 cdx ply. You want to make sections that will contour with and support the broken or loose areas, and basically staple the cdx to the existing trusses. If the bottom cord of the truss is also bowing down, I straighten that with an 8' 4x4 covered with carpet and jacked up against the truss to straighten it before securing the plywood to the trusses. There are a zillion details (yeah, not quite) and exceptions, depending on what you are up against. So think each plan through carefully or ask questions.
I hope this helps.