If inherent randomness exist in quantum mechanics, what then of eternalism implied by relativity? I am nothing but a curious layman so don't go too technical on me.
First of all, I am well aware that a lot of people consider the question of determinism vs indeterminism to be unsolved and others claim it is solved. That is not what this post is about, so let's not digress into heated debates over the validity of the interpretation I am asking about here.
Now if something like the copenhagen interpretation or grw is true then there is inherent randomness in nature. Before measurement, there is no truth about what outcome will be. There is no mechanism deciding that the infamous Cat will die or survive. There is no fact until measurement is made. I.E. the future is 100% open.
But how can that be reconciled with the view we get from special relativity? That there is no difference between past-present-future?
If I have understood the consequence of relativity correctly, that means that the "now" we experience is basically just a slice within a huge block of time. So the past is as real as now and the future too. And if that is the case then the future has to be fixed, otherwise what sense would it make to say that it already exists?
 A: You state that the following is a view we get from special relativity:

there is no difference between past-present-future

This is, to be frank, a sufficiently vague statement that it's hard to evaluate.  I would say, however that in my opinion, it's quite misleading.  As an example of why, consider the fact that massive objects in the real world are modeled to move along what's called future pointing timelike curves.  This basically means that they move in such a way that they're moving forward in time (or into the future if you will) and that they're moving slower than than the speed of light.  From this point of view there is a pretty clear distinction between past and future.
You also say that

the future has to be fixed, otherwise what sense would it make to say that it already exists?

I'm not entirely sure what it means to assert that "the future already exists," so it's hard to comment on that.  However, the statement that "the future has to be fixed" is really quite a loaded one and sounds a lot like you are trying to say something about determinism.  In particular, if I had to make a guess at what it means, I'd guess that you're espousing the idea that the state of the universe evolves deterministically in time, a statement which I think we all know is quite difficult to either justify or refute.
A: I think this is your argument distilled (correct me if I'm wrong):A: If the future exists it must be fixed.
B: The future exists.
C: Therefore the future is fixed.
Points A and B are both open to criticism. The whole argument seems predicated on the idea that it is meaningful to apply the word "exist" to the future. The problem is that in ordinary English "exist" is a present/past tense concept. I have no idea what it could mean applied to the future. This is something to argue on the philosophy stackexchange, or maybe english. Even if you argue for B there is still no reason A must be true. Future and past are different in relativity. The division of spacetime into past and future lightcones is invariant. It is also worth mentioning that there are perfectly consistent deterministic versions of Quantum Mechanics (they have other features that make them unpalatable, but they exist). So there are options of interpretation here which don't seem to be decidable scientifically. This definitely isn't a physics question.
By the way, if you're getting this interpretation of relativity from that Flashforward novel, well - the "physics" in that was pretty hammy. (Though not a bad read.)
