"but what if the student could confidently have expected to get their best mark of the entire degree on this one exam paper? "
You can compute the weight of this exam compared to the overall degree mark. That will hence be known and become part of the process of assessing what should be done (and the university, faculty and school should have standard procedures in place to deal with such circumstances). So, for example, similar procedures as are followed by students who submit mitigating circumstances (illness etc during exams).
Frankly, the mark for that ONE exam is only going to make a difference to the overall grade if they are close to a grade boundary anyway. In such cases, I would expect that the boundary region where cases are looked at would be widened, OR all people who sat that exam could have their boundary decision looked at. In cases where such students did fall just below a boundary, we would ask, was it the mark on THAT exam that was the problem, or were other marks at the lower grade. Because frankly, if you could see that 5 more marks, say, on THAT exam would have put them over the boundary, they'd possibly get it, especially if the advice according to the rules was that we should be lenient at grade boundaries for students who did that paper.
Then the external examiners would have their say as to what they thought of that process.
Or the person who set the paper in the first place could re-assess their mark scheme based on the difficulty of the new paper without the additional material. Which would then also be looked at by the external examiners and the exam boards.
As regards complaints -- frankly, there aren't any grounds for a complaint in this case. It was probably an administrative error of some sort (and I'm sure someone will be looking at exactly what went wrong in this case and whether or not procedures need to be changed or tightened to try and reduce the chance of this happening.) It was probably nothing at all to do with the lecturer concerned, because once exam papers are handed over to exams admin staff, we have nothing to do with the rest of the process until scripts turn up for marking. Even if an examiner had been abominably stupid and failed to include extra sheets/booklet/pages with the required appendices (not clear from OP what the nature of this material was), then I would have expected the internal moderator to have spotted that it was impossible to answer the question without additional material that they had not seen. Just as if I wrote an exam question that said, referring to Figure 2, when there was no Figure 2 on ANY page of the exam script!
As regards any supposed delay, I would assume that what happened was what might happen at my uni. Any query about the paper from the exam room has to go to the lecturers from the module who set the paper (who are supposed to be contactable DURING the exam, and this is also something that admin staff check before exams, in terms of where will you be and what contact number etc etc). If there was supposed to be, say, an extra appendices booklet/formula sheet, I would expect this to be LISTED on the front of the script. When it was realised that it was NOT there, I assume admin exam staff went desperately hunting for the envelope.............And then would have had to ask the lecturer if it was possible to at least partially answer the paper without it. Whilst also frantically explaining that, yes, they had looked, no one could find it etc etc.
The point I was aiming at originally was that there only would be grounds for complaint if there were procedures set-down for what to do in such cases, and the school or faculty or university ignored them. Or if there were not procedures and different schools applied different ad hoc procedures made up on the spur of the moment and you could show that some schools would have been disadvantaged by this. So, say, someone who got struck on the head by a meteorite in a physics exam would come out with a poorer result than someone who got similarly hit during an english exam.
What often surprises students, compared to A-levels, is that there is NO remarking, and that the academic judgement of the person who taught and wrote the exam and marks the papers, is unquestionable! There is no further expert authority, we are the experts on what we teach! But, yes, if a certain lecturer is held to be just plain abysmal, useless, a bit crap at lecturing etc etc, then student comments and complaints, and the success or otherwise of students in that module will be looked at, and conversations will be had. We have various procedures to peer-review teaching quality, we sit in on others lectures, we see what advice we have to offer about how things could be improved (I say lectures, but of course we have various modes of delivery, lectures, seminars, practicals, examples classes etc etc).
Because we all recognize that the quality and the content of our teaching and our methods, and how we listen and reply to student comments about teaching impact on the quality of students we get in the future.
Stuff happens (fire-alarms, administrative errors, printing errors, collapsing ceilings etc etc) during exam periods. You can never eliminate such events totally. Which is why we have procedures in place to deal with such events, and as long as procedures are followed and are fair and reasonable, and as long as we do strive to try and reduce the incidence of avoidable errors, then what more do people reasonably expect?