WASHINGTON — Then-second son Hunter Biden coached then-Vice President Joe Biden’s press secretary on how to respond to media questions about him joining the board of Ukrainian natural gas company Burisma Holdings, emails reviewed by The Post show. The May 13, 2014, exchanges between Hunter and Kendra Barkoff, which have not been previously reported, form the basis of a complaint sent to the Justice Department on Friday alleging that the Biden scion, now 53, violated federal law by failing to register as a foreign agent. “In advising the Office of the Vice President how to respond to press inquiries about...