Yes an open box should be fine, but I would separate them. Keeping him from her physically for a while will likely help her hormones calm down, or at least he won't be riling her up.
Calcium for eggs actually comes from the long bones in her body, so it's imperative that she get a lot of calcium ALL THE TIME, not just when laying. But I wouldn't over do it or you could cause a whole bunch of other problems. If she is eating her cuttle bone and her greens then adding a supplement could be dangerous.
She may or may not "let" you take the egg. You may have to gently move her with a cloth or a rolled up newspaper.
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