TxB, I would recommend replacing the screws that are leaking with the next size larger screws. The rubber grommets will last longer than any sealant you will put on. Take a cordless drill and just replace the ones where a problem leak is. Or you can get some elastomeric coating and coat the screw heads. Get it tinted to the color of your roof panel.
Now for the lecture, screw down metal panels with exposed fasteners are not meant to be roof panels over living space. If a good high temp underlayment such as Tamko "Metal and Tile" underlayment is used it will protect against leaks for a longer period of time. Heat will melt ice and water shield. Be careful on metal roofing, a 7/12 pitch is really slippery. Tie off with a line if you can and good luck.