Earl is not a single high. The last time he made a PBU or INT on the sideline that Sherm didn't tap to him was the INT he had in the playoff game against WAS. He is a cover 3 FS and, for the most part, he doesn't have to worry about the sideline. As far as the FS position goes, his job is easier if not different than a cover 2 or actual single high (cover 1) safety. Kam also played deep more last year, whether that be robber or lines up as the FS, than he has any other year. Even before last year he played deep quite a bit and has played it very well. Playing cover 2 for Sherm would also play to a lot of his strengths. He isn't the best athlete out there, but he has basketball length and that could be maximized in that 10 yard area that a cover 2 corner covers.
Cover 1 or man 1 is a real single high type of defense and will either leave you with a spy, lone mid zone, or extra blitzer in the short area. Depends on how you call it. Man coverage usually comes by way of a cover 2 man, 2 man under, or 2 deep man. Cover 1 is a Chip Kelly defense and it requires that your defenders are either better athletes than their opposition or they are that much smarter that they are a step ahead. As Chip found out, it should be used sparingly in the NFL. It isn't college football where there is some great, massive disparity between players and that's something he really needed to learn if he's ever going to be successful in the NFL. The latter version with everyone in man with the exception of the two deep safeties is much safer as far as the deep game goes, but you can still be cut up underneath as the offense can dictate matchups and isolate better offensive players against slower defenders, which we've seen at times in the case of Wright, in particular.
When it comes to man coverage, the defender is in a reactionary state 95% of the time unless he watches enough film that he recognizes the formation the offense is in as it pertains to situational football. In a purely reactional state, the defender must be either that much better of an athlete to negate the timing of a ball arriving as the receiver is turning his head or he has to know it is coming. When we talk about having these better QBs who are patient enough to dink and dunk against a cover 3 that almost makes them, they will also surely recognize man coverage with motion and start calling natural "rub" routes or pick plays for very easy completions.
There isn't an easy answer. Cover 3 will do the job well most of the time. When it doesn't we'll have to mix it up a bit and try to disguise and surprise. Brady, for instance, will kill you if he sees man coverage.