How, though? Assume a particle has some Flatten() method that is called only when it interacts with another particle. Until this is called, certain data points (let's say, anything not related to it's movement through space) could be either undefined or out of date.

When this function is called, it causes some algorithm to set up these data points. You can extend this to treat entanglement as copy-on-write style branching, where entangled particles A and B have some subset of their properties stored in the same physical memory, but calling this methods causes writing to those properties, and, thus, when the function is called for particle A, a copy of the properties is made for both A and B, and the evaluation is done for both.