I'm just commenting to let you know the final outcome of the thing, in case someone experiences a similar situation in the future.
In the end, I decided to contact ETS with a somewhat aggresive email, informing them of the effects that their negligence and demanding that they assumed responsability for it. In particular, I asked them to look for the exams more throughoutly and, in the case where they couldn't be found, either pay for my expenses for the retake in Portugal or establish a test centre in Madrid where I could sit the exam. Actually, I threatened them with taking legal action against them if my demands were not satisfied. This was kind of a bluff, btw, but hey why not?
It took a bit long for them to answer back, but they actually found a test centre in Madrid where I could take the makeup exam. The only issue is that such centre could only offer the exam on January 19th or 26th, which is 2 and 3 weeks after the original makeup date, respectively. Of course, considering that my test scores would be delayed even if I tested on January 5th, an additional delay of 2 weeks minimum could ruin my application, so I was a bit concerned about this.
For this reason, I asked them if they could guarantee that my scores would be published within 1 or 2 weeks of the test date, to minimise the disruption. Alternatively, I was also happy if they paid for my expenses for the January 5th exam in Portugal. At this point, the ETS officer I was talking with was not very clear on the details, and even sent me contradictory emails (on one, it said that I would be refunded for my expenses for the October 27th test NO MATTER WHAT, while on other, the officer offered to refund for the January 5th exam if I took the exam in Portugal - quite messy in the end) so I asked her to clarify. And...
It turns out that the exams actually appeared (quite suspicious, if you ask me, but I got a 870 - 89% so I'm not gonna complain) and the results were published on ETS website yesterday morning, so there is no need for me to retake the exam (in case you were wondering, it is not offered anymore). Also, they will send the reports to my institutions tomorrow so that's a happy ending!
That's why I feel that if this happens again, you should really press them and be aggressive (you know, not impolite, but definitely firm). There's no guarantee that the exams will be found (or the scores made up), but at least you can probably manage to get some better conditions, like retaking the exam in your hometown or covering for travel expenses, as I did.