on most drawings
depicting hidden diameters is normally done, as you say, in dotted lines and 10 inch diameter can indicate the bollards width at its middle section which, in this case, it does. Yours on the model
you are build
ing from the 1930s will not be prefabricated steel i.e. welded, they will be cast. The British shipyards were rather slow on the uptaking of welding steel and it wasnt until after the 2nd world war that we really got into the swing of things - long after the Americans, the Germans and the Japanese had been doing welding ships before the war. Three photographs which show you the type of bollards and the top of a bollard and also can show you the physical size of them.
As far as the mushroom vents go, according to British Steam Coasters by C.V.Waine, there is a small drawing which shows a true mushroom shaped ventilator on a ship - also in the book from Norman Ough he has a set of drawings
of them, although they were mainly aimed at Royal Naval ships, they would also be found on a merchant ships.