"Se considera solamente el supuesto de que siendo el propietario una o varias personas físicas no residentes.......".
I would use "here" for "hereby," and "fact" for supuesto but Quieta is right. And note how she reverses the order of the clauses. You can say, in Spanish "Sonó el teléfono," but in English you must say "The telephone rang." It is often a good idea in translation to reverse the order of the clauses in a sentence in order to protect the cadence or rhythm of the target language or clarify the meaning.
You could have said. completely literally:
We now consider only the fact of the owner's being one or several non-resident natural persons,..."
"Only the fact that one or several non-resident individuals are the owner(s) is here considered"
respects English word order much better. There are no rules for this; you develop an "ear," but Spanish tends to put the emphasis at the end. "¿Qué sono: el timbre o el teléfono?" "Sonó el teléfono." "¿Sonó el teléfono"? "El teléfono no sonó. Fue el del vecino." English tends to put the emphasis on the subject, which usually comes near the beginning. "Concluido el asunto..." "The matter having been concluded..."
Thank you very much, that has helped me alot, because I was finding the word order rather difficult as there are so many different ways in which one could express this sentence. I have taken your advice and it has served as a great help. Many thanks for your help.