Re: concentration of CO, and being heavier than air:
Suprisingly, CO is not heavier than air, and it disperses fairly evenly in a nitrogen gas. For this reason, groups like NFPA and UL certify hanging CO detectors at ceiling height. Even the US Fire Marshals Association, who initially criticised combo detectors, have now reversed course and endorse them.
However, there is plenty of academic concern about this on the web. While the densities may be the same, CO cools (and falls) at a much faster rate than nitrogen. In a small test chamber, this may not be pronounced, but in a large open room, CO can pool and concentrate at higher levels close to the floor.
The debate whether or not combo CO/Smoke units are smart and effective is a hot-button issue with the fire experts. The strongest rebutt to those concerns seems to be "Well, people are unlikely to buy separate CO sensors, and having a combo unit is better than nothing." You may draw your conclusions from that...