OK, just a tip. Before posting, read what you've written so it's understandable, not just a stream of consciousness.
Righto. To tackle your question of an person from Iberia applying to Qantas.
He/she would not get a start unless they had permanent residency in Australia.
Unless he/she was being hired for a specific role that no one else in Australia could do, they would not get a start. Yes, QF is a strict seniority airline. It doesn't matter what you have in your log book, you will start as a SO.
Most countries want you to do the hard work of getting visas for permanent residency yourself as well as the licence conversion. In other words, you have to qualify. For sure, the Asian airlines will generally sort that out but if you have something dodgy in your past, you won't be able to stay. Note, I am talking about Cathay, Dragonair etc, that's not a starting point for most people.
I would concentrate on getting a licence that gives you the greatest opportunity to work in the short term. Africa or Europe is where I think you have the greatest opportunity (assuming you do have British citizenship as well as SA citizenship). Also, don't always equate cheap with bad, expensive with good. Get the best value you can.
Good luck.
Remember, there are no guarantees