I have re introduced a flock to an older bird who survived a night time attack that left her last man standing. I waited to introduce a new batch in. I think that she may be fine to introduce sooner, my time line was dependent on me needing to hatch out a new batch of chickens. When they were 4...
Its likely just something you missed when you placed the egg inside the incubator. Did you see if there are any cracks in the egg? if there are no cracks, I would leave it in.
That is completely normal! They tak a few days prior to digest the yolk fully. I know it can be nerve racking when you've come this far! Just keep calm and carry on!