Drunk monks and the 5th precept I have heard that the 5th precept was not a core of Buddhism, it was created later by the Buddha after some monks returned drunk from alms because they had something with alcohol (accidently I believe)

My first question is: Is this story true?

Many Buddhist school claim that the problem with alcohol is that it can cloud the mind and make it easier for breaking the other (more important) precepts. Based on that some schools are ok with lay people drinking in moderation, with the commitment of not getting to a point of losing control of their acts or doing something they will regret.

So, is drinking with moderation (socially) breaking the 5th precept?
I believe this is the story you are referring to. It's a great story explaining the origin of the monks' rule to avoid alcohol. It also shows a bit of the Buddha's sense of humor.
This is from &quot;The Bhikkhus' Rules: A Guide for Laypeople&quot; http://www.accesstoinsight.org/lib/authors/ariyesako/layguide.html

&quot;The drinking of alcohol or fermented liquors is [an offence of Confession.]&quot; (Paac. 51; BMC p.402)
The origin-story concerns Ven. Saagata who conquered a fierce naaga — a type of serpent with magical powers — by his meditation-developed psychic powers. The townspeople heard about this feat and wanted to make some sort of offering to him, upon which the 'group-of-six' bhikkhus impudently suggested that they all should give him alcohol. When he arrived on his almsround every household offered alcohol and he finally collapsed, drunk, at the town gate and had to be carried back to the monastery. He was laid down in a stupor with his head towards the Buddha but in his drunkenness he turned around so that his feet pointed at the Buddha.[97] The Buddha called attention to his changed behavior, remarking that he certainly could not oppose &quot;even a salamander&quot; in such a state.
The Buddha also said:
&quot;Bhikkhus,... there are these four stains because of which samanas and brahmans glow not, shine not, blaze not. What are these four? Drinking alcoholic beverages... indulging in sexual intercourse... accepting gold and money... obtaining requisites through a wrong mode of livelihood.&quot; (A.II,53) (AB)

Again, this explains the monks' rule regarding alcohol. Regarding laypeople and the 5th precept, I'm not sure I understand your meaning of the significance of something having been created &quot;later by the Buddha&quot;. Perhaps a Buddhist scholar could identify the timeline of the 5 precepts; but my simple way of thinking of it is that they were created by the Buddha during his lifetime and whether it was in the first years of his teachings or the last; it was a valuable teaching to help lay people make their lives simpler (less suffering) by avoiding bad deeds. And avoiding alcohol is helpful for many people to stay mindful enough to observe the other 4 precepts.
Whether observing the 5th precept means avoiding even a drop of alcohol or just avoiding drunkenness or &quot;heedlessness&quot; by laypeople varies by tradition.