I understand that there are Eastern Orthodox priests who allow Oriental Orthodox to receive Holy Communion?
Would this be strictly forbidden and against the rules, or if not, then what is the logical answer to the dilemma that the two Churches, EO and OO are not united, but OO are in some cases allowed the Sacrament?
A priest who did so would be in error, unless for some unforeseen reason, in a pastoral situation,( I really can't think of any possible reason, but the Bishop has the responsibility of making the decision), he had the permission of his Bishop to do so. If he was doing it without his Bishop's knowledge and permission, he would probably learn very quickly what the words "Spiritual Court" really mean, once his Bishop found out.
I'm not really sure what you mean here, but intercommunion between EOs and OOs is a well established fact.
I'm not sure what you mean here by "an established fact", but this is not exactly true. I am merely an OO (Coptic) catechumen, but even I know that communion of EOs in our churches is by no means the norm. To the extent that it happens, it is in response to local pastoral situations, but does not establish precedents. For instance, it is allowed that a mixed Coptic-Greek couple who are married in Alexandria (home of both the Coptic and Greek patriarchates in Egypt) may be allowed to intercommune, but that does not mean that I can go to the local Greek Orthodox church and commune there. In fact, the priest who serves us here in Albuquerque has been quite clear that he will not commune EOs in our church. I haven't asked him, but I don't doubt that the priest of the Greek church would say the same about us. There's simply no need for it, even if both (or some subset of both) might want it. We are, sadly, not unified yet.
Contrast that with the situation back in my home area of N. California where there are no OO churches at all and you'll find that lots of Eritreans and Ethiopians commune at the local OCA and Bulgarian churches. Again, it is a response to pastoral need, and God bless the Bulgarians, Russians, Greeks, Serbs, Arabs, and others who have accepted the Habesha Orthodox. It's a wonderful thing that they are responsive to brothers and sisters in need, and it brings the best food in the world to the otherwise dolma and spanakopita-heavy Glendi festival that they hold every year at St. Seraphim of Sarov Church. </shameless plug>