1.) Were an updated General Theory capable of making verifiable predictions about the international economy developed and written so that it stands alone, without requiring prior understanding of Keynes's General Theory, generally be used as a text for macroeconomics in undergraduate and graduate courses?   (i.e., would it be commercially viable?)

What you're looking for is a Feynman Lectures on Economics, so to speak, which as you know would have a cult following and hence be commercially viable but would be very unlikely to be used in any courses, graduate or undergraduate.

2.) Could such an updated General Theory be used, (or have such updated theories been used), to generate computer based economic models with significantly more predictive and explanatory power?

I suspect such (explanatory, I don't know about predictive) models could already exist, based mostly on Leontieff's Input/Output analysis. Also things like the model developed by the Club of Rome for the Limits to Growth.

3.) If, as seems likely, both 1. & 2. could be done, what does the fact that they have not been done show?

Are they really likely and to what extent? Bruce says downthread

the General Theory is about those aspects of a monetary production economy for which you can draw a general theory ... rather than, as some people misread it, a "theory about everything in general".

Also, Economics is more like evolutionary biology and ecology and an evolutionary discipline cannot be predictive except in the short term. In this case, the rate of economic innovation is much faster than the rate of speciation which makes "the short term" really short.

When the capital development of a country becomes a by-product of the activities of a casino, the job is likely to be ill-done. — John M. Keynes