Well, I haven't done the reading test, but the text looks fine to me. My overall comment is that apart from a few minor errors, it seems a valid test. Your question was about native speakers and the test. Well, IF they know the format, and have a reasonable understanding of what examiners expect, and can spell, they SHOULD pass the test. THEY WOULD HAVE NO TROUBLE AT ALL IF THEY WERE TELLING YOU ABOUT THE QUEEN AND HER FORMER RESIDENCES. They would happily change tense and and tell you where she HAD BEEN. HAS BEEN, WAS GOING TO BE, and all that. The problem is that many native speakers are LAZY when it comes to grammar, and get into bad habits, or have ways of saying things that are very regional, or are trendy! SO, many native speakers would get a lot of it wrong, I'm sure. But that does not mean they can't speak English! They are PERFECT speakers of English, but they do it their way! And maybe they can't spell! And maybe they hate exams. YOUR job is to ATTAIN grammatically correct English, and pass exams to PROVE you can speak English. Most English, Canadians, Scots, Americans, Australians, Welsh, Irish etc. don't have to bother very much. So their attitude is very different. Most of us will NEVER be asked if we know the difference between the past perfect and the present perfect. We just go by the SOUND of the words. Back to the test: you will no doubt have found it tricky. But it is a valid test, and will sort out a good C2 from a struggling C1. Get hold of some PROFICIENCY course books. You'll find very similar material in them.

It looks like they took the passages from texts that they didn't copy directly (there are edits between the originals - https://www.royalcollection.org.uk/visit/clarencehouse/about/who-lived-in-the-house, http://www.princeofwales.gov.uk/the-prince-of-wales/residences/clarence-house, https://www.washingtonpost.com/world/africa/protests-over-black-girls-hair-rekindle-debate-about-racism-in-south-africa/2016/09/02/27f445da-6ef4-11e6-993f-73c693a89820_story.html, and https://www.theguardian.com/books/2016/aug/21/farewell-to-ice-peter-wadhams-review-climate-change). Their edits to the original add some confusion. There are a few points where I might make a different choice, and there are times when they add mistakes in (the ones MoiraWendy points out are correct in that original and Queen Elizabeth's name is not misspelled) The language in the roleplay and the "speaking - pictures" sections is atrocious. The person who wrote those should not be teaching English or judging others' use of the language.

Yes, but whoever wrote it should be shot. For example, they habitually use a comma where a full stop is required. That is a shooting offence in anybody's book.

Part Two - the vocabulary. 12. This needs an 'AN' in the text for 'inquiry' to work. Without an 'an' it would have to be 'inquiries.' 15. students's ??? I think they mean 'students' (but allegations is correct). The errors are minor, and could be typos. Again, you should have no problem IF you have a very wide vocabulary. But without that very broad vocab base - it will be very difficult. But it is meant to be. I'll have a quick look at the reading comprehension in another mo....

Part One: There are a few mistakes in it (apostrophe missing, comma in the wrong place) but actually, it's not difficult choosing the correct verb forms. You SHOULD be able to do the GRAMMAR part without too much difficulty. I'll have a look at the vocabulary in a mo....