Then worlds in which I choose Y are logically incoherent
From an omniscient point of view, or from your point of view? The typical agent has imperfect knowledge of both the inputs to their decision procedure, and the procedure itself. So long as an agent treats what it thinks is happening, as only one possibility, then there is not contradiction because possible-X is always compatible with possibly not-X.
Those are the conditions under which counterfactuals are flat out impossible. But we have plenty of motivation to consider hypotheticals ,and we don't generally know how possible they are
You are assuming a very strong set of conditions..that determinism holds,that the agent has perfect knowledge of its source code, and that it is compelled to consider hypothetical situations in maximum resolution.
They are not logically incoherent in thenselves. They are inconsistent with what actually happened. That means that if you try to be bundle the hypothetical,the logical counterfactual ,in with your model of reality, the resulting mish mash will be inconsistent. But the resulting mish mash isn't the logical counterfactual per se.
W can think about counterfactuals without our heads the exploding. That is the correct starting point. How is that possible? The obvious answer is that consideration of hypothetical scenarios takes place in a sandbox.
Under determinism, you should be a nonrealist about real counterfactuals, but there is still no problem with logical counterfactuals. So what is "the problem of logical counterfactuals"?
Until you hit a hard limit, like lack of resources.
Without some assumption similar to “free will” it is hard to do any decision theory at all, as you can’t compare different actions; there is only one possible action.
Under determinism, there is only one actually possible action, and that doesn't stop you comparing hypothetical actions. Logical possibility =/= real possibility. Since logical possibilities are only logical possibilities, no sincere assumption of real free will is required.
Since you are invariably in a far from omniscient state about both the world and your own inner workings, you are pretty much always dealing with hypotheses, not direct insight into reality.
this “what would have happened” world is logically incoherent.
There is a logical contradiction between the idea that your actions are determined, and the idea that you could have acted differently under the exact same circumstances. There is no such problem if you do not assume determinism, meaning that the "problem" of logical counterfactuals is neither unavoidable nor purely logical -- it is not purely logical because a metaphysical assumption, an assumption about the way reality works is involved.
The assumption of determinism is implicit in talking of yourself as a computer programme, and the assumption of indeterminism is implicit in talking about yourself as nonetheless having free will.
A purely logical counterfactual , a logical counterfactual properly so-called, is a hypothetical state of affairs, where a different input or set of preconditions is supposed, and a different, also hypothetical output or result obtains. Such a counterfactual is logically consistent -- it just isn't consistent with what actually occurred.
According to counterfactual nonrealism, there is no fact of the matter about what “would have happened” had a different action been taken.
People calculate logical counterfactuals all the time. You can figure out what output a programme will give in response to an input it has never received by looking at the code. But note that that is a purely epistemological issue. There may be a separate, ontological, not epistemological issue about real counterfactuals. If you have good reason to believe in determinsim, which you don't, you should disbelieve in real counterfactuals. But that says nothing about logical counterfacuals. So long as some hygiene is exercised about the epistemological/ontological distinction, and the logical/real disinticntion then there is no problem.
The apparent nondeterminism is, then, only due to the epistemic limitation of the agent at the time of making the decision, a limitation not faced by a later version of the agent (or an outside agent) with more computation power.
Note that problems agents have in introspecting their own decision making are not problems with counterfactuals (real or logical) per se.
This leads to a sort of relativism: what is undetermined from one perspective may be determined from another.
It doesn't lead to serious relativism, because the perspectives are asymmetrical. The agent that knows more is more right.
A problem that comes up is that of “spurious counterfactuals”
A "spurious" counterfactual is just a logical, as opposed to real, counterfactual. The fact that it could never have occurred means it was never a real counterfactual.
Equivicaction is using a term in different senses *during the course of an argument"..that is under conditions where it should normatively have a stable meaning. It is still the case that some words are ambiguous, and that recognising ambiguity can solve problems.