It's not really immutable as you can change the parameters of an entangled pair. You just can't communicate any information by doing so, because you need a classical signal to make sure you don't read one of the particles the wrong way.
I could be WAY off, but if locality isn’t entirely true, and the “read success” is 33-67%, doesn’t that still leave quite a bit of wiggle room for communicating information in some fault tolerant method?
You get correlations - you can "understand what you read" once you have the measurements from both entangled particles, so you need another channel of communication (with the associated delays) to get that information.
One side doing their interaction may cause a "spooky action at a distance" (according to some QM interpretations), but if you have only one side of readings and don't know what the other party measured in their interactions, you can't tell anything about what "the other side" did, so it does not help communication at all because you still need to transmit as many bits in a non-quantum way until you can do anything.
Correlations only but no useable communication. You can both make a decision on the same random info that isn't determined until later when you are apart, but can't know anything other than that if they followed the plan they made their choice based on the same later-determined random info, correlated with your random info.
If they didn't follow the plan and measured orhogonal/same (can't remember which) spins, then your results are uncorrelated but you can't know until you meet back up (maybe barring superdeterminism that is also accessible to the individual).
If we agree before parting that one of us is going to Alpha Centari and the other is staying on Earth and going to assassinate either the President of Russia or America depending on the observed state on an entangled pair of particles, once I reach the star system.
Doesn't the traveler have more information than anyone else on ship about whether an assassination attempt was made in Russia or America? and have it faster than the speed of light? We don't have it with certainty, but we have shared knowledge that is unknowable to others and instantaneous.
I think you would have a shared private piece of correlated information between each other that wasn't determined until you made the measurement (though maybe no joint reference frame to say who made it first), but you can't choose what it was (communicate with each other).
The universe either had to break the light barrier to make the measurements correlated (predetermining the outcome isn't generally possible because you could choose how to make the measurement based on another quantum measurement from something outside of the other participant's then-current light cone), or make the same choice through superdeterminism (the other measurement and all others were predetermined too and exact simulation of entire future universe's measurement decisions was shared between every particle when they were within some distance at big bang or something). But even though the universe broke the light barrier, you yourself aren't able to use it for communication.
In the many-worlds interpretation you've both branched into the same branch of the multiverse, but couldn't choose which branch. You do have private knowledge of which branch and the consequences of that, assuming you both followed the agreed on procedure.
I think you can use what you are describing in a series of correlated measurements to set up a provably secure one-time-pad, and then do secure classical communication with it. But you don't communicate the actual bits of the pad, you just both get correlated ones.
You didn't gain any information after parting - you'd "know" just as much if your compatriot on Earth had given you a sealed envelope that said "Russia" or "America" inside.
You can take actions that will later turn out to be correlated with each other, but you can only find that out once you meet up again, bounded by the speed of light.
