Your analysis considers only diffusion of CO2 whereas my experience is that convection is also important, and running a heater is in some configurations a great way to induce convection.
I sleep in very small room -- only 45 sq feet and 45 * 8 cubic feet. I don't have a CO2 meter, but I've been sleeping in the room for decades, so I have many data points, and I am pretty sure I can tell (by noticing that my thinking is duller and stupider than usual the next day) when CO2 levels were too high overnight.
No matter how high the CO2 level are in the room, 20 minutes of having the door wide open will bring CO2 to levels I cannot distinguish from outdoor air even if there is nothing (no wind, fan or heat source) to cause convection. Here we see diffusion in action. But at night, I prefer to have the door open just an crack (about an inch). Configured that way, CO2 levels clearly get too high if I rely on just diffusion: I need convection to keep CO2 levels healthy, and a heater running inside the room provides sufficient convection (as does a fan pointed at the crack of the door).
There is one exception: when the air outside the room is hot enough (and here I guess I should mention that the door to this very small room basically opens to the outdoors) then the heater might not provide enough convection (and even if I could be assured that it did, it would make me too warm on very warm night). I.e., my system of running a heater and having the door open only a crack should not be relied on to keep CO2 levels low in very warm weather unless you want to do research I have not done (e.g., with a CO2 meter).
I should mention that this very small room is unusually airtight: even the round holes in the metal boxes inside the walls that contain the electrical outlets and light switches have been sealed. I mention this because it has made it easier for me to compare the effects of convection to the effects of diffusion because when there is only one opening (a door in my case) in an otherwise almost completely airtight room, the presence of a wind (and there is some wind or breeze most times in most places on earth) does not induce significant convection across the doorway.
I used to rely on an alarm clock to wake me up every 105 minutes to air the room (which takes 6 minutes on very cold nights and 10 minutes on the warmest nights). (When sleeping this way, the door is completely closed during the 105-minute intervals of sleep.) I no longer sleep this way because I've become less tolerant of having my sleep interrupted every 105 minutes. I mention this to underline the fact that I have plenty (decades) of experience with diffusion and convection in the context of CO2 and sleep.
(The 6 to 10 minutes of having the door wide open does not reliably reduce CO2 to levels indistinguishable from outdoor air, but I consider it good enough, and I wish to reduce the duration of the discomfort of lying in bed with the door wide open.)