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Hi,
1) In short, I can roughly say that 건설 is to a job if 공사 is to a work. 공사 is used for precise processes as I understand. 건설 is more likely to mean "to build".
2) -는가, -ㄴ가/-은가 are the endings for interrogative sentence and used when a person asks to other person of the same or slightly younger age. However, they are also used in the relatively formal articles to unspecified mass/multitude, such as columns in newspapers. I guess it's because the writer doesn't have any idea of the readers' ages.
Best, Hang
