I'd keep forced air vents & esp. returns well away from an open Fp. Make sure you seal the ducts with mastic and test with Duct Blaster and a Blower Door test for house. Min. 0.35 ACH. Per ASHRAE 62.2-2004, all mechanical ventilation must have makeup air unless a blower door test proves sufficient passive leakage. Seal the top of the house tight as Tupperware with makeup air introduced as low as possible. Keep foot traffic away from Fp as much as possible. Make sure the chimney is at least 2 ft. taller than the house's equivalent stack ht.
There's a lot more but if you're hell bent on putting vents near a Fp, I'd start with this list to prevent smoke spillage. However, since you sound more concerned with sucking up a smidge of heat to redistribute it, I'd highly recommend plenty of good smoke alarms along with some professional grade CO alarms that exceed UL 2034 in sensitivity.
Good luck--I think you're going to need it.