how to interpret this diagram?

[grovenor-2685]

I doubt you will make any more sense out of this unless you can pin down the dates when data for each of these plans was compiled and relate that to signalling alterations.
Rgds

[Noel]

John, I was looking at the 6" maps for the periods before and after Shotton station was built, which, incidentally, seem to show the signals there before the station. If so, it suggests that the primary reason for them being there is operational, not because of the station being built.

Moving the advanced starter for CQ2 doesn't change the situation in terms of accepting trains if the clearing point for the Queensferry homes is not clear. However, giving Queensferry outer homes a mile away does, as trains can then be accepted by Queensferry whatever the situation at that station. Given the low power to weight ratio, compared to modern trains, even a short, but in the context of steam trains, fairly significant, gradient like this would probably have the effect of slowing train speed somewhat. On a busy main line with close headway this could cause bunching and delays in clearing the Queensferry section; outer homes might make the signalmans' train regulation job a little easier. Just a theory.