Things don't change.

This is simply false, as a statement, so I won't treat it on it's own.

It has an intended interpretation that isn't false, which I referred in the following statements which you've accepted. (It's more of a summary than a separate point.)

The future is what it is given your actual decision,

True. But making choices requires that one accept that one doesn't know what the future is, nor does one know what one's decision will be. It requires the use of "if... then" thoughts, or counterfactuals.

Yes. If there is a (logical) fact in what your actual decision is, say it's actually A, and you are uncertain about what it'll be, then the assumption A=B is logically false, inconsistent, even if you don't know that it is. When you reason about what happens if A=B, not knowing that it's a false statement, you are reasoning from a false premise, and everything logically follows from a false premise. This is the relevance of this description.

all else is fantasy

Emotional dismissal, not an actual point.

Not emotional. What else is there? There is reality, and then all the thoughts you can have to reason about reality.

A good counterfactual should be logically consistent. It isn't the real world, but the real world isn't the only logically consistent possible world.

If there is a fact of the matter of what your action is, then assuming a possible action that is not actual is logically inconsistent. This is normal. If you are considering something that is not the real world, you need to explain what relation it has to the real world, and how this particular not-real-world is different from all the other not-real-worlds, and what this not-real-world actually is, especially if it's inconsistent, but even if it's consistent, there is still the same question of what privileges it, since it's not the real world, and real world is what you want to reason about.

perhaps morally important fantasy whose structure we ought to understand, but still not the reality.

Dismissal, not an actual point.

It's a point that it's unclear what relation is there between the counterfactuals and reality, given that counterfactuals are usually not the reality.

EDIT: So, which of those are you claiming I contradicted exactly?

You referred to "slightly modified version of the past", and modifying things is starting to consider things other than reality, where it becomes unclear how considering those not-real things helps to understand reality. (Uncertainty is a much better concept than change in this context.) I would further qualify that you can't change your notion of reality without moving away from your original notion of reality, and thus conceptualizing something other than reality, when what you wish to understand is reality, and not this not-reality you've constructed by modifying the concept.