Hear me out...
It sounds like you have the gist of the full sync down, but make sure you do a couple things before assassinating Pitcairn.
1) Wait until guards are as far away from him and his horse as possible.
2) Try to make sure they are looking AWAY!
Even though you're pretty much doing everything else right, how far away the other guards are from Pitcairn DOES make a difference.