I’m no expert on fire shelter, but here’s some physics:
1. Air in an inground cellar will be much cooler than outside air. Vents/doors all open to ground level above. Cool air wants to sink and will stay in cellar. Warm/hot air wants to rise and will stay above outside. I believe the risk of toxic gases entering cellar from ground level will be minimal. Limiting factor will volume of air in shelter.
2. You can reduce duration of shelter time by reducing fuel (large trees, buildings) in the immediate area. Small fuel load will burn out quickly and allow you to come out fairly soon.