I've never heard of this happening to an LS or any other car, really. I'd be more prone to suspect that either the connection at the fuse block, where it melted, was a loose or dirty connection or the original alternator failed in such as way to degrade the fuse block which only continued to get worse. It's pretty well known that a loose or dirty connection in an high current application will get hot and can eventually cause connectors, wire insulation, housings, etc. to melt and sometimes ignite. It sounds like that's probably what happened here and unfortunately it was not fully diagnosed early in the car's life. I think you'd have a really difficult time creating a case against Ford or against the shop that put the "warning sensor" in. I really have no idea what the "warning sensor" would be unless it was the voltage regulator or alternator. A corporate lawyer would likely tear your case up if you went in and tried to claim the "charging system warning sensor was replaced."
-Rod