How would these "safeties" work in your mind?A JAG officer sitting by the President and signing off?A "distinguished" triumverate of say,M. Gorbachov,Jimmy Carter and Kofi Annan required to agree?I just don't see it your way,unless you simply mean the President would have to justify his actions after the fact,that is obvious.
That's not the way it works. The football is a communications device and has no decision making capability. It has authentication codes. Those codes, when received and authenticated by the launch officer, basically order the silo to launch on a specific target.
The "launch order coming from the President is not going to have to be justifiable in terms of immediate threat." Rather, the launch order coming from the President (via the "football" with appropriate authentication codes) is all that is needed to command a launch.