Mark your eggs with a pencil, don't use ink. Egg shells are porous. I usually scribble all over mine. It's better to isolate the hen. I think half an hour is okay, but not sure she should stay off much longer than that if it is cold outside. I guess the other chickens laying in the nest could have warmed the eggs back up?
Sorry, that's a myth! I use a Sharpie marker. I mark the entire fat end of a shell under a broody to note which eggs she has been sitting on, if others add to the nest. Marking the end makes it easy to not have to roll the eggs around or move the hen very much. I know right where to look.