I'm a fourth year trying to finish his PhD.

In my university, each PhD students needs to pass through 2 exams meant to provide a different opinion on his work work throughout the process.

I failed my first attempt, and and the examiners suggested that I got demoted to a Master's status. Following this, I have taken every suggestion from the examiners, I have changed and developed my dissertation in the directions they suggested, I have written a completely different submission, one that I found positive and that even my supervisor liked (the other one resigned a month before the deadline).

At the end of the Viva I have asked one of the examiners to let me know the result as soon as possible, because the first time they let me suffer for a month. He called me in, sat me down, and said they were sticking to the Master proposition. Apparently, there is nothing wrong with my submission (which was called "perfect", and "marvellous"), and I am the issue, and my ability of carrying the argument home in the Viva.

We have discussed for 4 hours, during which I understood what kind of answer they expected (i thought my work was under examination, and so I offered data in support of my statement, whereas they were looking for evidence of critical thinking and argumentation), and at the end of our discussion, the examiner was convinced I was owed a new possibility. he has since been trying to obtain a third submission, which would be uncommon but not unheard of.

I am still waiting on the report, and I know a lot is going on behind the curtains, but it still kills me that I might be failed right now. The research is basically done, the writing process is at an advanced status, and they are stopping me because I was unable to understand the subject of scrutiny at the viva. I am setting my mind to an appeal, in case I was not given the third submission, but I am at the end of my rope.

I am very sorry to hear this and it does not sound fair at all. If they don't give you a third chance you should appeal. As after the first viva they gave you feedback and you delivered the changes. Then they failed you a second time for a completely different reason and admitted they failed you the first time for these previously unspecified reasons. Therefore their feedback was not appropriate and purposefully mislead you. You should be allowed to try the exam again now that you have the true

I think you should contact the students union and the postgraduate support team (or similar). As well as record all interactions with them. I wish you the best of luck.

I have received the document right before leaving to present at the most important conference in my field for the second year in a roll. which is befuddling.
I have been failed, the examiner did not even proposed the third submission.

The report is simply appalling and incredible: not only they state that I would have conceded that my dissertation could hold itself without the theoretical model - I did not, and it could not - but I am being failed because the examiners did not approve the theoretical model, the one subject I was told I could not be failed on: as i said, I needed to pass two exams before the end of the disseration, and the theoretical model was the subject of the first exam, which I passed. even if I wanted to cut it off, I could not. the examiners are setting themselves against the examiners of the first exam.

Iam looking for legal aid, but it is terrible.

I am very sorry to hear this. They should have criticised the work not the way you present it. I think they have got the feeling that your results were not based on a solid theoretical background. Some examiners like to see the PhD student provides very good reasons for choosing particular models/topologies.
I would advise you to appeal as I have read here several cases with successful appeals. Keep in mind that the probability of accepting the appeal is not particularly high. So hope for the best and be prepared for any outcome. I wish you the very best.

I'm struggling to understand why they told you that you could not fail the theoretical part of your PhD. Having a demonstrable understanding of the theoretical work underpinning your research is absolutely crucial. You cannot and should not pass without having that.

Who told you that you "could not fail this part" and do you have that in writing?