It is a fascinating idea, that a being from the "twelfth level" can become an avatara and incarnate on earth whereas a being from the "seventh level" cannot, but this idea doesn't fit into my belief system. I guess we can simply agree to have different belief systems.
This is certainly contradictory it seems, but this is the problem with combining Buddhist and Hindu teachings in a single discussion. An avatar is not possible in Buddhist teachings, for Buddha has said that a being cannot incarnate lower than his current attainment - for Buddha, it is a constant forward motion, although within each level of existence there are many possibilities based on karma here. Circumstance, for instance, in the human birth can signal former karma, but there is also the total possibility to escape what Hindu's had discussed as their caste system through the karma of this life. In Hindu thinking, your entire life is dictated by the former lifes karma, there is no possibility to elevate paste a certain point. Based on the totality of karma during this life, you may raise in the next or go backwards. This is a significant difference.
Of course, there is also the concept of the Bohdisatva, which is more aligned to the Hindu concept of avatar perhaps - and also closely assossiated in Buddhism with the concept of God in other traditions. This being is not awakened, he has chosen not to become enlightened or achieve nirvana because he wants to assist beings again. It is essentially the return of a being very close to attainment.
If the Kingdom of God can be experienced here on earth, then they are definitely talking about enlightenment (but not nirvana). I have heard the idea that Moses' Promised Land was not heaven but nirvana, and this makes a lot of sense to me. And as you seem to be suggesting, both the teachings of nirvana and heaven were later merged into a single of concept of heaven and then taught to the masses.
The Bible states many things on the subject, for instance:
Luke 9:27: "I tell you the truth, some who are standing here will not taste death before they see the kingdom of God."
Luke 17:21: "nor will people say, 'Here it is,' or 'There it is,' because the kingdom of God is within you."
Certainly, the first appears to be saying it is possible here on earth, while the latter states just the same as many great masters, that it is found within.
Do you follow the idea that nirvana is nihilism? If nirvana is the extinguishing of a person, then how can that person go on to higher levels?
No, nirvana is not nihilistic. A person moves to a higher level of existence through karma, but in each realm they must still attain nirvana to escape the cycles of birth and death. No level of existence is permanent, none escapes the natural laws of motion.
How do you distinguish between enlightenment and nirvana?
I do not, which is making this discussion complicated, lol
There is only reason: to come back out of compassion. Any other reason is meaningless. Avataras only make the huge sacrifice to incarnate because they do it out of compassion.
This is not post-nirvana, and certainly not post-parinirvana. Their act of compassion is during life to hold back their practice, it is not a choice after the death of a Buddha. I have also discussed occult practices to accomplish the same, but again these are taken during life.