Utawarerumono

-The all beginning is when Iceman got shot when he discovered something he shouldn't. The blood was absorbed by the fossile. How long has the time passed until he wake up again with some strange mask on his figure and became a subject? He has lost his memory from his accident.

Unknown, considering all life on Earth was wiped out and those scientists no longer had a immune system capable of handling virus ... must been a VERY long time, likely over 1000 years.

-When all the subjects were freed so as iceman and mikoto, it was said they were iceman's children since they were created after iceman. So all the ppl we met through the game story on earth are "descendents" of those subjets?

Well yes and no, they were based on Iceman DNA but they build based on the fact Iceman HAD a immune system capable of withstanding living on the surface.

They were as much descendant of him as we are all brothers at best, from a genetic POV.

-In the end... Who is Iceman and mikoto's child? How come the ring she recieved was given to eruru ?

Iceman and Mikoto child? unknown ... good chances of being dead since the child was a baby and he turned everyone into red slimes, except for Mitsumi that never says she did anything about and of course they already killed Mikoto with him being next.

-Mitsumi #63 was a failed subject, that has no material form... it's only spirit that loves iceman and consider him as it's father, right?

No, Mitsumi had a body since the scientist said they had to restart to experiment on her again after Iceman escape, also before that they disassembled her body but keep her brain ... in the game time period Mitsumi spirit lives on the bodies of her descendants, the Onkamiyamuka waiting to awake.

-When Iceman got caught again and lost mikoto, he went mad and transform himself (against his will?) to destroy everything around granting wishes there and there. He wanted to be stoped, to be killed. Mitumi appears to grants his wish, but finally he got sealed and sent on earth. Time passed and there was when he met dii for the first time. He possessed him and got split for some reason...Since they can't be together, hakuoro(wounded?) was thrown away of dii's body and went in the forest were he met eruru and made a contract in order to heal himself.... through this, he lost his memory, gained back his human form and there starts the game?

... I know its confusing but no, Iceman and Witsarnamitea are different persons (after all Iceman entered a contract with Witsarnamitea) and even if Iceman wanted to be killed he was also not shy of twisting the scientists words to give then a fate worst then death.

Also this is were the game goes give a bit of a plot plot hole ... Diy was possessed at least 10 years before the game timeline started since he was the one that given the Avu Kamuu to the Shakukopuro but the game also gives the idea Diy going "missing" was a relative new event so were lies the truth?

Diy found Witsarnamitea, Witsarnamitea enters a contract with Diy, "Diy" starts the conflicts around so they can evolve, years pass and then Witsarnamitea enters a contract with Eruruw and game starts with her taking care of Hakuoro that is also Witsarnamitea.

Iceman is Witsarnamitea body but so is "Diy", if Diy dies its not a issue since Witsarnamitea can just get a new one but Iceman is a different matter since it contains the real body of Witsarnamitea (the mask), not that is a issue because neither of then can actually be killed, if "Hakuoro" "dies" he simply goes back to sleep and "Diy" wins that round until Iceman reawakens and forges a new contract.

Confused? well if you still are ... lets say its a very complex concept to understand.