Newbee,
Any changover to the chimney would be questionable at best to the
inspector. Even now it is a questionable setup considering the
sharring of chimneys. Im afaid I would recommend the ventless or
electric water heater. Maybe your installer of the last one can cut
you a deal for a tradein since your recent water heater is so new.

PS. on the smoke exchange between chimneys and vents this would work
to seal the fireplace chimney airtight downstairs http://www.chimneyballoon.us