1) Given a particular feature on the board, how can I figure out if it is available on a classical 8051.
a) why worry about it, if you happen to use something that is not in the "classical" you have learned something anyhow. b) if you REALLY need to avoid that, go by "the bible", if it is in "the bible" and not in the processor (this does not apply to the f12x) the processor is not a derivative but a deviate and should be avoided totally.

2) This board has many additional features. Learning all them now doesn't seem to be a good idea. How can I develop something that uses only the features of classical 8051?
This keeps my learning curve short and my code compatible with all derivatives.
If you go by the Keil "getting started guide" and other Keil stuff such as "blinky" you will stay with the classical and go through it by very good guidance re soft. For an expanded view use the tutorial here at 8052.

3)My time spent on this is my personal time. Ofcourse, like everybody elses', it comes at a premium. So, how can I quickly figure out the details. I know: THE DEVIL IS IN THE DETAILS.
How can I do something realistic soon.
DO NOT expect to become fluent in 3 days, that is NOT going to happen. Learn to crawl, THEN learn to walk. see the answer to 2)

Erik