Yep, the koreimu was placed inside marisa, then made pregnant through a paste injection. The premies suddenly being conscious is just plot magic.
The prems were dunked in the parent Marisa's sweet paste, so they woke up to eat all that. From the inside out.
Updated
Look, the little ones have woke up.Ooh! Still alive, still alive.You've got no choice.Gahahaaha.
Don't make me say it again.Pii~!!Young yukkuri are only able to give birth to disabled yus.Yupuu~Yupii~YUGYAAAAYuyuu?Yuu?Don't underzdand... just... stop...And they'll be in your body... In other words what do you think will happen when they're born in the middle of sweet sweets?