Johnny asked:”When a zygote is impregnated with a gandhabba, any effort from this point forward to terminate life, is it considered as committing panatipata?”
Yes. Once the gandhabba (or the patisandhi vinnana) descends to the womb and “takes possession” of the zygote, there is another human life in the womb. Any attempt to kill that life (via abortion) is killing a human being; see, “Buddhist Explanations of Conception, Abortion, and Contraception“.
“Another question is, can a gandhabba attain sotāpanna magga and/or phala?”
That is an interesting question. In principle I don’t see why not. But I have not seen a Tipitaka account of a gaandhabba attaining magga phala or the Sotapanna Anugami stage.
“Can I view the gandhabba state akin to a traveller (after acquiring a new patisandhi viññāna) who is inside the airport transit lounge, waiting for the right time to enter the holding area and eventually board the plane that will bring him to the next destination?”
The situation is better described by the case of a human operator inside a totally enclosed military tank. Here the human operator is analogous to gandhabba, and the military tank is like the physical body. The human operator cannot see, hear anything outside the tank, without the help of video and audio equipment mounted on the tank. In the same way, gandhabba cannot see, hear anything without the help of the physical eyes and ears on the physical body; see, “Our Mental Body – Gandhabba“.
– Of course, the gandhabba can see, hear by itself once OUTSIDE the human body. That is what happens in Out-of-Body experiences;see, “Manomaya Kaya and Out-of-Body Experience (OBE)” and “Gandhabba Sensing the World – With and Without a Physical Body“.
Another related point is that manomaya kaya is the more generic name for the “mental body”. All living beings have manomaya kaya. Gandhabba is generally reserved for the mental bodies (manomaya kaya) of humans and animals. That is because, those human and animal manomaya kaya can inhale scents (gandha + abba, or “taking in odors”) and become a bit more dense.
P.S. I have revised the post in question, “Human Life – A Mental Base (Gandhabba) and a Material Base (Cell)” to make this clear.