The law specified that the president had to have been dead for at least two years. It also specified that the presidents had to released in the order in which they served. This causes the problem in that Carter could not be shown because he had not died, but if they depicted Reagan then the Presidents would not be released in the order in which they served (Because they would be skipping one.) As Jbuck said they bowed to the money and to the popularity of Reagan.
While the legislation required that the President had to have been dead for two years it placed no such restriction on the First Spouse. If Carter had died by the end of 2014 both Rosalyn Carter and Nancy Reagan would have been eligible to have the image of a living First Spouse on a coin. If Hillary had been elected in 2008 and died by 2014 then yes Bill would have been eligible IF They decided to skip over G W H Bush, Bill Clinton, and G W Bush to do a Hillary President dollar.