1 - The Hongmeng organization is basically a sect of cultivators. The "true purpose" of the sect is that they're sealing some creatures(I can't remember what they are exactly), the more important aspect of this seal is that Zeus is sealed inside. The cultivators have to funnel their energy into the seal on a regular basis in order to sustain it. The cultivators need to be at least at the spirit transformation realm in order to power the seal.The gods are only weaker than the cultivators because the environment of the Earth doesn't enable them to reach their max power. Or something like that. There's also a difference in power amongst the gods, Zeus is the strongest followed by Athena, the other 10 are a fair bit weaker than the two of them.Zeus and Athena = Jade Purity Divine Thunder
Transcend Tribulation cultivators at their full power.The other 10 = Supreme Purity Divine Thunder
2 - His grandfather dosn't have any other identity. The woman who is always with him is a spirit transformation cultivator from a prominent family in Hongmeng. There's a small story behind her but i can't remember the specifics, and she dosn't really matter anyway.3 - Honestly, it's kind of a cluster f'k. The gods are actually aliens from mars trying to revive their god clan, in order to do so they have to basically terraform the Earth which would kill everyone on it. The old immortal just wants to find a successor to take over his position as guardian of the Earth so he can leave and go fly around the universe. MC is caught in the middle. MC does fight the gods at the end, and "wins" I guess threw the power of love yay.4 - He is a pawn of Athena, she sets him up as an enemy of the mc to make the mc grow stronger. He gets power I think from the god stone.5 - Just don't pay attention to this.
I would not call him a carefully honed puppet. I would say that he's unlucky that he's talented enough to replace the Old Immortal.He fights the other gods because in order to revive their clan they need to plant their mother Gaia which is a tree. The gods are fruits born on that tree. In order for Gaia to grow and produce new god clan members it will do something, I cant remember exactly what, something involving the sun, anyway it makes the Earth enter an ice age which in turn would kill all life on the Earth. Then the god clan will be all that's left. So in order to save all his family and friends he goes to stop them.
Houtian - Xiantian - Spirit Transformation - Transcend Tribulation - "Immortal"Transcend Tribulation is a real cluster f of a realm. It's divided into 9 stages but there's a bunch a crap that determines the cultivators power in this realm aside from their level.