Yeah the only fear I have is exploding eggs? Will duds explode if I leave them in there the full 21 days?
If bacteria gets inside any egg it can go bad, whether it is developing or a dud. If bacteria does not get inside, the egg will not go bad. Whether it is a dud or not has nothing to do with it possibly exploding.
Incubation temperature is the perfect temperature for bacterial growth so if bacteria gets inside expect it to multiply. Those don't always explode, they can seep a foul smelling liquid. You can usually smell if an egg is going bad before they start seeping or explode if you sniff them. That rotten egg smell is pretty horrible.
The vast majority of duds or developing eggs under a broody hen do not go bad. The vast majority of duds or developing eggs in an incubator do not go bad. The bloom is extremely effective in keeping bacteria out. The best way to prevent seeping or exploding eggs is to leave the bloom intact. Don't wash or sandpaper the eggs. Don't set dirty eggs. A light dusty "soil" is OK but clumps of dried poop or mud can give bacteria a route inside. And keep your hands clean (especially not oily) when handling the eggs.
When an egg goes bad it is bad. That rotten egg stink is horrible. If any of that gets on another egg it can and usually does infect it. It can ruin hatches. I don't take it lightly but the only time that has ever happened to me was when an egg broke under a broody hen and the egg material contaminated the other eggs. That was a ruined hatch. The only time I remove any eggs from the incubator is when I candle them just before lockdown. The only ones I remove then are the ones that are totally clear, have no chance. That's not because I'm worried about them going bad, that's just to make more room in the incubator for hatch.
I don't set dirty eggs and I don't compromise the bloom. I understand an egg can go bad but it's not something I worry about.
we read that bantams hatch closer to day 18-19 but we have a bantam roo and standard hens. We know the eggs get fertilized because we are the bullseye every morning for breakfast. Anyways, when does lockdown happen for bantam/standard crosses?
I don't keep bantams, but if I did I'd go into lockdown with bantam eggs at the standard time for chicken eggs until they proved to me that they do regularly hatch early. I've read that they can possibly occasionally might sometimes hatch early, not that they always do. I hatch different sized dual purpose eggs and have paid attention to size and when they hatch. Size has not been a factor with my eggs. If you want to lockdown bantam eggs a day early you can, it probably won't affect the hatch. That way you can determine for yourself if yours do actually hatch early. My dual purpose eggs often hatch early, not because of size but because of heredity. Whether they are in my calibrated incubator or under a broody hen they are often two full days early.
Besides, that bantam rooster has nothing to do with when the dual purpose eggs hatch. He doesn't have anything to do with what size eggs that dual purpose hen lays. His genetics will affect what his daughters' eggs do, but mating a hen does not affect any of that with her eggs. Treat them like what they are, dual purpose eggs.