I have one more question. Since the Coptic Church is not heretic (and nor are any other jurisdictions heretic), I am free to attend any ritual in any Orthodox Church, right?
Unless you participate in sacraments.
So, as a (Greek) Eastern Orthodox Christian I am not allowed to participate in Oriental Orthodoxy's sacraments?
As a general rule, no, since we've not established official communion yet. But much depends on how your bishop interprets this rule.
What about the rest jurisdictions? Can I take part in a sacrament in an American Orthodox Church? Or a Russian one?
As long as you're an Eastern Orthodox Christian in good standing, you can receive the Mysteries in any canonical Eastern Orthodox church.