Thanks for showing this. I was kind of along the lines of "assumption" when I posted that, but I can get on board with what you are saying too. I was kind of relying on the fact that the specific gravity of CO2 is heavier than air which will also help keep the air out of the carboy even after...
I was doing a little looking around and this is a neat idea if you are worried:
https://www.homebrewtalk.com/f163/too-much-head-space-secondary-carboy-316272/
Also, if you look at the specific gravity of CO2 it is heavier than air, so you shouldn't really have too much a problem because...
If you keep a tight bung with a filled airlock you shouldn't have any issues. The CO2 will push outward until CO2 development ceases at that point you still have an air tight seal thanks to your airlock.
Subscribing and really looking forward to seeing what will come out of this. I remember first trying Harvest Dance and I loved it. You have inspired me to also give this beer a go soon.