I have a feeling if we get a change in what we consider the present state of history in Lost, that people will feel Faraday was wrong, or lied, or the writers were confused, or lied, or cheated, and many people would possibly not like that the show would violate what they see as confirmed canon. But to me, such a thing, like young Ben staying dead, wouldn't violate anything. For one, they have shown us small violations all along. Stopping death for a day, meeting someone in the past, though it already happened differently before. So really, they could have characters say anything and that doesn't mean you don't believe your eyes.
So, firstly, what is the future that can't change... is it 2008, or 2080? Did it already change? If Ben stays dead, I said if, how do we know that won't correct time?
Secondly, "what happened happened" is true in multiverses. Multiverses branch, and also remerge (correct themselves, you could say). There are two branches, one where Faraday contacted Desmond at the hatch, one where he didn't. Both happened. History does not change in that model of time travel. It's worth noting that model is and was favored by quantum physicists like Richard Feynman. Not a shabby model to follow. And if you are being subtle, statements like "whatever happened happened" would still be true.
In this model rules about what you can change, how long it takes for time to correct, and that sort of thing, the patterns of corrects (like what is the relationship between the ways the deaths alternated for Charlie while being saved), make sense since the universes are connected like a river delta structure. You are not totally unconstrained, there is a limited volume of possibilities.
Given ALL that, we only experience one, and this fact can also make us think of one of these threads of time as the "real" one, and in that sense, I think Lost uses a notion of this thread as special. This real thread is where all the alternates correlate, so when desmond had two experiences in 1977 he was able to merge them together. He's special, we couldn't do that, we might get headaches and die from having that. We need to have our thread just through one universe in common with the people we interact.
A more liberal interpretation QM-wise is that what you know about people is correlated in your time line, whatever you don't know and can't confirm (know indirectly by knowing people that know or sources of information that would tell you) is uncorrelated, for you the other person lives all the possible time lines. For them, it never seems that way, but it seems to them that you live that way. Lovely relativism and multiverity.
Ok, I went beyond what I think Lost is doing a bit, understand, but it relates and at least explains why I like discussing what it means for history to change, and what sort of model Lost uses for time travel.
BONUS QUESTION: I really love the type of time travel Desmond was doing, and the other guy from the freighter, psychological, where your mind travels back and forth to consciously control yourself at other times. That's a really pretty believable types of time travel, somehow, and the mix of memories and confusion make powerful tools to investigate psychology. I'd like to see more about that sort of thing. I also think it's clever enough that they can use it for a few wildcards to fudge a few continuity stretches.