1-Isn't communism collectivist in nature?
Communism means collective ownership and democratic control of the means living. Ergo, yes, it is collectivist.
and if so, isn't communism by proxy against liberty since collectivism in itself is at odds with liberty?
Not at all. Collective ownership will ensure personal freedom.
2-What do you mean by libertarian communism? is it the same as libertarian socialism or anarcho-syndicalism? and most importantly how will a libertarian communist society look like? other than pre-civil war Catalonia, could you give any other example of such societies? can we regard the kibbutz as an example?
We haven't reached it yet. It is the goal of history.
Kibbutz were/are racist institutions that have nothing to do with socialism-communism.
And socialism-communism can't be a island in a sea of capitalism.
I know about workers co-operatives and direct democracy and freedom....etc but how can you abolish private property and enforce collective ownership without a strong state?
It can't be enforced. The goal of communism and anarchism is the socialised ownership of the means of production under democratic control. That is the basis of the new society.
Personal property will still exist. No-one will want to nick your toothbrush.
But I don't have diagrams of cookshops of the future.
Last of all, I believe that even if anarchist communism can be implemented on a small scale I still have serious doubts about a nation surviving under such a system, what can you tell me to convince me otherwise?
I can't, because nations must be dissolved.
Nevertheless, it may be that the world will not be de-territorialized in one instant. So, the democratically controlled legal institutions will exist in territories for a while, mayhap.