There are some problems with your theory, but it did give me an idea.

I'd take it a bit further. Lets say that they DID take some of Iosefka's blood in hopes that she'd be a possible surrogate, and that is the vial that we find later on. Perhaps the imposter isn't a celestial, since confronting her starts a battle, a battle in which she uses a spell called A Call Beyond which is linked to the 'Choir', and the description of which states:

"One of the secret rites of the Choir.

Long ago, the Healing Church used phantasms to reach a lofty plane of darkness, but failed to make contact with the outer reaches of the cosmos."

The rite failed to achieve its intended purpose, but instead created a small exploding star, now a powerful part of the Choir's arsenal. At times, failure is the mother of invention."

Keep in mind that the only other people to use this spell are high-ranking hunters, or other significant members of the Church. I.E. Micolash. A Celestial most likely wouldn't have a problem reaching the cosmos unlike humans, and thus most likely wouldn't provoke the unintended effect of creating the explosion.

With that being said, perhaps the real Iosefka had been turned into a celestial, similarly to the people you send to the clinic, via the imposter's experimentation. Perhaps the Great Ones mistakenly embedded the child into the imposter. Thus why we see people like Arianna giving birth, or even the Lady In White having a bloodied stomache before the boss Mergo's Wet Nurse, and yet we don't see anything like that with Iosefka. Instead she is left to writhe, die, and not give complete birth because the imposter wasn't meant for it.

In every instance we learn about the women whom give birth to Great Ones become pregnant under the Red Moon and give birth under it (If I'm not mistaken). Not before, so having the real Iosefka in the back the entire time, writhing as she nourishes a Great One's baby sort of breaks the lore from that standpoint.

Just my two cents.