Would this be a worthwhile project to undertake?

I'd like to see a project involving this. Would be extremely interesting and revealing perhaps.

I wonder at what point in the gestation process, the embryo becomes an ISBE?

I don't know but -

"Only 22 days after conception or 5 weeks and 1 day gestation, the embryo’s heart starts beating. This heartbeat will not stop until the individual dies."

https://lozierinstitute.org/fetal-development/week-6/#:~:text=oxygen%20to%20survive.-,At%206%20weeks%20gestation%2C%20the%20embryo%20has%20a%20steady%20heartbeat,stop%20until%20the%20individual%20dies.

In this interview, Terrence Howard is telling that he remembers being in the womb at the age of 2 or 3 months:

➡️ https://www.farsightprime.com/forums/general/68693-terrence-howard-interview

See also:

➡️ https://www.farsightprime.com/forums/general/68541-dt-logistics-and-a-disturbing-possibility

So few years back my ex girlfriend thought she was pregnant, we were already separated but still met each other sometimes, few weeks after, she felt something that lead to the conclusion that she probably had a spontaneous abortion since she never felt nothing after that. Few years later i started to delve into the experiencing of being a father, there was so much energy around that subject for whatever reason that i even got a lucid dream experience with hybrids, an old lady hybrid delivering me a baby, a hybrid children, the level of love that i felt there was overwhelmed, i even remember my mother trying to understand what was happening while she was seeing a baby with strange look in his face, "it's a hybrid baby mom, we talked about this, there are other people apart from humans remember?!".
After that episode i started to experience some strange episodes in my home, things disappearing, displacement of objects, one after the other almost like poltergeist stuff. Later, a friend of mine invited me to do some readings, she is a psychic / medium, and without me saying anything she started to talk about a kid that she was seeing, " i have here a beautiful kid trying to speak with you, he's saying he's the reason why you are forgetting/lost some objects in the your home, he's trying to connect with you, to call for your attention, he was the one coming forward with you and your girlfriend but the path got closed. He is however your master..." meaning at that time i was doing energetic work, more like Reiki and so on, and for those that understand a facilitator has someone on the other plane helping energetically.
Was an amazing experience all of this, specially the felling of fatherhood.
So since time is an illusion/real, and is happening here and now and from my experience from this and other friends of mine, things are happening not only on this level.

https://en.wikipedia.org/wiki/Beyond:_Two_Souls

"The game features Jodie Holmes, one of two player characters. The other is an incorporeal entity named Aiden: a separate soul linked to Jodie since birth."

Spoiler:

If I remember correctly, Aiden was a twin of Jodie that died in the womb or at birth, and then somehow got connected to her. Very intriguing plot.