Baldrick.....I have a slightly different take on your Plan set
The oval shape superimposed AFT of the engine roof level oval simply represents the shear or angle of the chimney toward the AFT end of the vessel. The blackened area is simply the only overlapping profile between the two and is of no consequence in any build
The yellow lined oval is the actual inter funnel casing, the green lined oval is the outer shell casing of the funnel. The is an air space between the two was also used for modest heat exchangers or water heaters
If weight was not an issue, an internal oval formed pasti-card representation [sprayed flat black on both sides] would be scale like
You mention that Cervia was oil fired, but was capable of a relatively simple modification to be coal fired. So the designers included a coal fired chimney
Naturally a hand fed coal fired boiler required a larger volume of air draw & hence the larger surfaced area [and taller] chimneys over a pressurized oil fired boilers etc
Derek