Is this true that only at the Quinisext Council it was allowed to draw Jesus on icons as a man, and that before that time Jesus was only depicted on icons as a fish or a lamb? I've looked up the article on the Quinisext Council at the Wikipedia, but it doesn't say anything about that.
Quite early in Christian history the cross (without a human form on it) became a symbol of Christianity and appears to have become a superstitious symbol as well. Heathens recognized this seeming superstitious behavior, so that by the time of Tertullian heathen called Christians cross-worshippers.
You are correct though, from what I can tell. It was not until the fifth century the crucifix began to appear:
To answer the question if before this time the human figure was ‘not allowed’ is difficult to answer. Possibly since nobody ever thought of doing it, it was simply not done? We can’t really say it was not allowed unless people were trying to do it and some authority forbade it.