That got me thinking too. If Legs and Head are the same person, we know for sure that the one who did him that is not the Shichibukai, since Head doesn't know the name of the guy who cut him (Meaning that he doesnt know who he is, so he can't possibly know he is a Shichibukai).
If you read last week's chapter, you will see that Legs ask the SHs if they are ALSO allies of the Shichibukai, so what I am assuming is that Trafalgar Law is the guy behind the Samurai's status but have nothing to do with the PH guys, M and the kids.Probably the Samurai started to attack Law's men when they met in the island mistaking them with some allies of the Shichibukai, they called for help (The call that the SHs intercepted) and Law came and defeated the Samurai, leaving him in that state The Shichibukai the Samurai is talking about should be M, who kiddnapped his son.
Did it occur to you or anybody that legs might be his son?