I see these two words used interchangeably, but I don't think they are interchangeable. Here was the title for an article in yesterday's FT: "Buying a property near London's Olympic site is a failsafe investment, right?" Seems to me that they should have used "foolproof." To me "failsafe" means that if you fail to do something, there is an automatic default to a safe condition.
I agree with you on all points.
I agree. Also, as I understand it, "foolproof" is an adjective that generally refers to procedures; "failsafe" is a noun that generally refers to Plan B (e.g. safety nets, belaying lines, lifeboats).