Here's what we are looking at: Shauna Hoare speaks of Becky in the past tense in interviews with the police in March. But Hoare is not charged with murder until months later.
What does this mean? There are several possibilities, and these are only a few:
1. The police didn't notice Hoare's use of the past tense and/or didn't understand its significance.
2. The police did notice, understood the significance, and:
2a. Initially believed that all this proved was that Hoare knew Matthews had murdered or killed Becky.
2b. Believed from the very beginning that Hoare was jointly guilty of murder along with Matthews but didn't think they had sufficient evidence to support bringing such a charge in court against Hoare until additional evidence emerged much later.
2c. Believed from the very beginning that Hoare was guilty of murder AND believed that they had sufficient evidence to support bringing such a charge in court, but the initial charging decision was made by the CPS against the advice or the input of the police investigators.
I respect your analysis and I have learned a lot about this case from your posts, which I always enjoy reading.
I think your line of thinking is certainly a possibility.
But there are other possibilities as well, ones which encompass both police understanding of the significance Hoare's remarks as well as the lack of an early murder charge against her. I think either (2b) or (2c) are reasonable possibilities.