The poor advocate fallacy

The other day while I was on the train home, I was pondering a few logical fallacies, as one tends to do from time to time. They are fascinating things and serve as useful tool to sharpen up one’s thinking. I know that I have, from time to time, been guilty of committing some of them, though I doubt I am alone in that. In this pondering, I found myself thinking of the following situation:

1. Person A is an advocate of proposition X. Person B is an opponent of proposition X. Or rather, they may be an advocate of proposition Y which is incompatible with proposition X.
2. Let us suppose that one of these propositions is true. By their mutual incompatibility, the other is false. For the sake of argument, we may assume that proposition X is true and that Y is false.
3. However, person A’s reasoning for believing proposition X is faulty. Precisely what the flaw is need not concern us. There may be a long chain of reasoning in which just one or two steps are in error. The chain of reasoning used by person A we will call P. So what we are saying is that P does not imply X. However, there may be another chain of reasoning which is correct, lets us call it Q, which does imply X.
4. What happens then is that person B notices the error in P and highlights it. However, they then assert that because P does not imply X and that person A has used reasoning P (without knowledge or understanding of Q) that X is false.

This is what I would call the poor advocate fallacy. A has not advocated a false proposition, but the fact that they used poor reasoning to get there has, in the mind of B, undermined their argument. Thus B is guilty of trying to take a shortcut and instead of using a valid line of reasoning to assert the truth of Y (and thereby de facto falsifying X), they use the falsity of the reasoning P to incorrectly conclude the falsity of the proposition X.

In thinking this through, I tried to look it up but could not find the fallacy named as such. So this is an open question to you – is this fallacy known by another name?

Or, and I cannot discount this possibility, is my reasoning faulty? If so, though, does that have any bearing on the whether or not my assertion that there is such a thing as the poor advocate fallacy? Because if there is such a thing and my reasoning is faulty, might a detractor to this idea be guilty of the fallacy themselves?

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Such are the musings that occupy my mind from time to time. The fact that this got published is partly due to inspiration from Revd Claire and her take on some much tougher philosophy than the simple logic I propose above.