Hi Jon,

Quite a mixture of ideas in your question.

• Feeding steam up the chimney to look good? Yes.
• Blast pipe? Probably not, they're used in a locomotive to pull more air through the fire when the engine is loaded. Good for making the engine go faster, bad for fuel economy.
• Condensers don't dry steam, rather they make it very wet by converting it to water. Condensing water creates a vacuum that's applied to one side of the piston while the other is pushed by steam under pressure, making the engine more powerful and economical. Condensers are controlled by valves in time with the piston. It's part of the engine rather than a simple add-on. Certainly add one for interest, or if modelling an engine that originally had one, but perhaps OTT on a Stuart.
• As you say a condenser can be configured to separate oil from the steam. This is important when the water is going back into the boiler as in a marine engine, less so when the boiler can easily be refilled with clean water. Glands, boilers, valves, and condensers in a steam engine can all be badly damaged by oil disintegrated by steam and boiling water. Marine engines were usually fitted with specialised separators to protect the delicate condenser. Elaborate precautions aren't so important or useful in a model. I like the smell made by steam engines: removing oil from the exhaust might spoil that.
• Passing exhaust steam through the feed water will economise on heat but as you say you have arrange the plumbing to deal with water, lots of it, condensing in the pipework. You don't want to impede the exhaust or blow boiling water about!
• Cooling steam in the feed-water before it goes into the condenser isn't a good idea because it reduces the vacuum and therefore the efficiency of the engine. No need to do it that way round because hot water from the condenser can be used to pre-heat the boiler feed.

Several reasons for making a Stuart. They make a good looking working display, in which case puffing steam up a chimney with a blast pipe is attractive. Not a good idea to fit a blast pipe if your interest lies in making a working condenser to increase the power and efficiency of the engine. And a condenser will remove the energy needed to work a blast pipe, and the amount of visible vapour in the exhaust, spoiling the good looks of a steaming chimney. Using exhaust steam to preheat feed water will economise fuel but its not a good idea if the boiler is filled with an injector rather than a force pump. (Injectors work best with cold water.)

Taken individually, lots of interesting ideas for experimentation, go for it. Applied together liable to be counter-productive.

Dave