Those are good questions. But it is time to look at those issues from a different point of view.
1. The difficulty in understanding the gandhabba concept lies with the mindset (mostly in the Western world) that is anchored on materialism. Starting with ancient Greeks, it was assumed that everything in this world can be explained with matter (objects that have weight and take up space).
2. However, within the past 100 years or so, after the scientists started probing the matter in detail, they have got into the “quantum regime” where the boundary between matter and energy disappeared. Now we know that matter can be converted to energy (atomic bomb) and that “invisible energy” can move matter. An example of the latter is the ability of scientists to drive robotic vehicles on the Moon from a laboratory on Earth.
3. So, why is it difficult to conceive that the gandhabba can move the physical body? If scientists can move a vehicle on the Moon from here (using pure energy as the intermediary), why can’t the gandhabba (which is essentially an “energy body” with very little matter) move the physical body from within the physical body?
4. The other question about the “instantaneous” movement of a gandhabba to a womb or the birth of a Deva in a Deva realm to take place instantaneously with the dying of a human on the Earth.
– There is a verified phenomenon of “quantum entanglement” in physics. In 1997 Nicholas Gisin did the first experiment with entangled photons, in which they were sent seven miles apart via optical fibers. If one of those photons was forced to change its spin (an intrinsic property) that INSTANEOUSLY changed the spin of the other photon. See, “Quantum Entanglement – We Are All Connected”
5. The rebirth process works in a similar same way. But the explanation is a bit deeper because it takes place in the vinnana dhatu.
– When the gandhabba of a human dies (at the end of human bhava) a Deva in a Deva realm is born instantaneously. Those are just two events on the same “lifestream”. The potential to cause a Deva life in that Deva realm may have been there for a long time (as that human did moral deeds and built up enough kammic energy in the vinnana dhatu). That is also the “kamma bhava” which turned to “uppatti bhava” at the moment of death of that human.
– For further details on vinnana dhatu, see, “Search Results for: vinnana dhatu nama loka”
It requires time and effort to understand these deep concepts. But it is fully worth the effort.