I think that as soon as you have swapped the false eggs with the real, you need to work as fast as possible giving Phaere the eggs and going to the Matron Mother. Unless someone can correct me on this....
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Melusine, Archbabe of the Order of the Holy Flame and the Laughing Hyenas, &
Official Entertainer Elf of the BG2 Bar
Your voice is ambrosia