The JHWH: So: Was the doctrine of trinity decided to be a christian doctrine by the same people who chose which books to include in the bible? I vaguely remember that both of these subjects were decided at the first council of Nicaea. If this is not the case, then when, where and by whom were these things decided?
Ummm! The trinity doctrine developed to deal with certain contradictions caused by Christian beliefs. There is no clear story of the development of Christianity, all we can do is read early writings and attempt to figure out what was happening.
For instance, Ignatius, the Overseer (Bishop) of the Antioch group of Christians was arrested around 110 CE, and sent to Rome to be tried. On the way he was greeted by representatives of various local churches, to whom he later wrote. Seven of those letters survive. The topics mostly deal with faithfulness and obedience . But from the viewpoint of JW dogma there are two interesting points in his letter to the Romans.It is clear from his that letter that he expected to be with Jesus soon after his death- there seems no expectation of a long sleep in death.
But the second point touches on your question. In ch.3 vs 3, he writes,
"Our God Jesus Christ, indeed, has revealed himself ..."
So, within a generation (at the most) of the death of the Apostles, a Church leader could call Jesus, "God."
However, that was just the start of a long journey to reconcile that idea with other Jewish teachings. However some scholars (like Daniel Boyarin) feel that the vision of Daniel 7 had already prepared the way for a concept of a senior God and a Junior God.
Boyarin feels that this idea came from the long intellectual movement within Judaism from polytheism to monotheism. ( See his The Jewish Gospels:
The Story of the Jewish Christ.)
I myself have the opinion, that the Hellenisation of West Asia introduced (if nothing else already had not) the concept of apotheosis, the elevation of a human to Godship (divine status). In Greek mythology, Hercules (Heracles) who had a human mother, but a God (Zeus) for a father, was apotheosised after his death and was escorted to heaven by the Goddess Athena. How natural for the gospel writers to describe their beliefs about Jesus, using commonly discussed Hellenic ideas.