when i use a carbonation cap i fill to just above the where the curve starts and squeeze the bottle til the liquid reaches the top. This both purges the o2 and gives room for the co2 to enter and have some headspace.
No it doesn't. You aren't drawing in CO2 when you release your hand from the bottle. All you've done is just sucked back in the O2 you "purged". Bottles don't breath like humans do.