It's hard to make the judgment call! Of the 2 chicks I've lost after lockdown, I believe they were both because I didn't act fast enough. We do our best and just need to love on the babies that we do manage to help successfully!
This is definitely what happened with my Mallard eggs! The last 2 eggs that were developing officially stopped though so my guess is that the longer the development is delayed in early development embryos the worse off they are. Alternately I've also had more developed eggs under a broody that she abandoned in 40-degree weather that were still ok after several hours of being chilled. In the end, I did try to help those chicks without an incubator and set up a plastic box with a seedling heat pad under it and an office lamp over it for warmth, due to my inexperience at the time being my first batch of eggs I wasn't able to save them but not for lack of trying!