I think that if I'm walking down the road on a hot day and get thirsty and then I decide to buy a soda then my purchase is of my free will despite being cause by the heat, cause by society teaching me about buying things, etc. And I insist that if you told a non-philospher I bought it of my own free will they would agree. So I think you're entirely wrong to say that something that is part of a causal chain cannot be called free.

Now, if you told a normal person that someone in that circumstance couldn't have done anything except buy the soda they would agree that would mean I didn't have free will but they wouldn't see why. To them the "My decisionness" and "Could have been differentness" are the same thing. But philosophically we have to choose one or the other and it's both more useful, in terms of carving reality at the joints, and in greater accordance with common usage to pick "My decisionness" as what we interpret "free will" to mean.

I'm sorry I'm not directly addressing your point about changing the past, but that's because I'm still rather confused by how it relates to free will. Certainly I'd say that nobody can change the past.