My only problem is that this usage of "should" in second conditional has not been mentioned in any grammar source. Michael Swan only uses it with "I" and "we". [. . .]
Do you know any reliable source in which this usage is discussed?
Yes, Freeguy, this British use of "should," which relates to "shall" in its first-person usage in British English, is generally used only in the first person -- and it is generally not used at all in American English. The last style guide in the U.S. to have recommended it without reserve was published in the nineteen-sixties.
The British usage of "shall" and "should" allows for changes in perspective, and it is still possible to conceive of such changes in perspective. If you and I were speakers who said things like "I should like a cup of coffee" (rather than "I would like a cup of coffee"), it would be grammatically possible for me to ask:
- Should you like a cup of coffee?
If you are interested, you can find discussions of this usage in the Fowler brothers classic, The King's English, published a century ago. There is also a description of perspectival shifts with "shall" and "should" in Wilson Follett's Modern American Usage (the style guide from the sixties I referred to above).