That sounds like exactly how the eggs should look at this point. And no worries--it takes a while for a dead egg to build up the bacteria loads that cause explosion--it's unlikely to happen even if they die right before or during lockdown.
Also, everyone's experience is different, but I open...
I think Day 18 would be June 11... but it's late and my brain's not working properly, so I may be wrong...
Not sure about humidity--I've only hatched chickens once--I usually hatch ducks. But I always leave the vent plugs out during the hatch. Babies need air even more than they need humidity...