Hope it's ok to chip in, but I'm just reading this and I'm so sorry to hear about this fiasco with the tests that has affected so many. As if they haven't got enough on with upcoming mocks etc and they will have worked hard to prepare for those tests.
Also, I wanted to say that quite a few subjects have done away with these admissions tests anyway - for
instance, Cambridge don't bother with what was the GAT anymore as they decided the results had no bearing on anything (inc degree outcomes). So for geography, it's just interviews, PS, references and contextualised grades / predicted grades (plus essays for some colleges). So there is no reason why other subjects can't do the same, particularly humanities and social sciences. I think also they dropped the Chemistry assessment at Oxford? So no reason why they can't make decisions in the absence of tests for other sciences.
I think the MAT may be more integral as maths is much less subjective, but they will just have to shortlist without it I suppose.
I also once saw a list of admissions test scores for a few subjects correlated to who was shortlisted for interviews at Oxford. I think it might have been on this site a few years ago when I had a DC applying. There seemed little correlation between those with high test scores and those shortlisted for interview - I was very surprised and wondered why they bothered with the tests at all, based on what I saw eg. people with a 2 or 3 were being interviewed while others with much higher marks were not. What did correlate though, was a high interview score to an offer. So even though they talk about a 'holistic' assessment, I think it's probably the interviews that hold most weight. That's my feeling having gone through this twice.