Hi Mahendran,
I think I can answer a few questions:
If the conditions are not good enough the gandhabba does not enter or will leave the womb.
The matching criterion is the matching gathi of both parents.
Bhava means there is kammic energy for a certain existence, e.g. as an animal. The bhava lasts until that energy is spent.
The gandhabba contains the kammic or bhava energy, wether the gandhabba is inside a physical body or not. Thus kammic energy is always needed to sustain the bhava. Even if the gandhabba cannot find a womb and has to stay in paralowa the kammic energy would run out within a certain time span.