What does “a coherent research question” mean?

You are right that I was using “coherent” partly to mean logical flow.

But there is a distinction.

Coherent

Coherent means that the different parts fit together logically and do not contradict one another.

For example:

Did legal emancipation create legally free labour, and did capitalism subsequently provide opportunities for some of those workers to improve their economic position?

That is coherent because the second part follows logically as the next historical question after the first.

Your sequence

1. Legal freedom.
2. Free labour.
3. Capitalist employment.
4. Working population.
5. Some upward mobility.
6. Some middle-class emergence.

Yes—this is a logical sequence.

But a sequence alone is not enough to establish causation. You still need evidence at the transitions.

So I would now say:

Your argument has a coherent causal sequence, provided each transition is separately supported by historical evidence.