That's not how some of us believe it works, as we tried to explain multiple times (pardon the phrasing)... You can't travel into your own past, you can only travel into the past of an alternate timeline.
Timeline A - Data's head isn't there, Picard and Guinan meet somehow, events transpire to lead the TNG crew to SF in the past. As far as the rest of this timeline is concerned those folks never reappear.
Timeline B - TNG as we know it. Data's head is there because Data and others from timeline A crossed into the past of this timeline. Once this timeline reaches the point of Time's Arrow, the TNG folks will go into timeline C, etc...
Of course the root problem with this particular situation is that it technically is a paradox and shouldn't exist in the first place. My understanding is that even most of those who believe time travel is possible don't believe that you can create a situation where something happens because it already happened.
I'm rather tired right now, so I regrettably may have botched things a bit in explaining it this time. But in an alternate timeline I got it right!