Porch swing mounting (also in SC) years ago: I could get into the porch attic from the other attic and slide a board, possibly 2 x 6 but I don't remember, over the roof rafters, spanning more than one of them. I would recommend a 2 x 6 anyway. Then drilled 2 pilot holes into the 2 x 6, going through the wood ceiling also, and screwed the mounting hooks into the board. Swing is still there.
In your case with the metal ceiling I would try to take down a section of the ceiling so you can access the rafters above the desired location of the swing. Get your boards to span 3 or so rafters. I think there is mounting hardware for outdoor swing sets which have some kind of bearing arrangement for the mounting chains. There needs to be a set up where you drill through the boards and fasten the supports with a nut and washer so as to distribute the load instead of depending on threads in wood to support the swing. I don't think we used that but may have used oak lumber which would not e as likely to allow the threads to strip out.
KEH