I might be misunderstanding you but the issue/structure you mentioned with "but such a feat" is also used in numerous correct official gmat question. I think the resumptive modifier, which is also correct, is a related cousin of that structure as well.
David@VeritasPrep wrote:You can start a sentence with a Gerund, and in fact it is done on official GMAT questions to confuse people. That statement is incorrect and it is the first time that I have seen of this one. Sorry for that confusion.
D is still the best answer. You have the feat itself at the end of the opening clause and after the
comma it begins "but such a feat" so this one has the best flow. If I were pressed I would pick D.
But let me say that we do have a problem here, in that the "incorrect" answers are not incorrect enough. A is not "wrong" enough and even choice C has some merit.
I would certainly edit this one if it came across my desk. It is the responsibility of the question writer/ editor to make sure that there is only one right answer.
D is still the best choice and has the preferred style, if not exactly the only allowable structure. You guys are correct, some editing needed on this one...