It's all very difficult.
Grade inflation is inevitable under these circumstances. Imagine you have 10 kids who you think are on course to get an A. In a normal year, one of them might have a bad day and end up with a B. This year, even if that happens in the last assessment, the school still has the flexibility to say "they've got A on everything else they've done."
I think that happens far more often than someone you thought was heading for a B pulling off an A on the day, so taking out the "bad day phenomenon" pushes grades up.
Then you've got half a dozen kids who you're sure will get either an A or a B, but it might depend on what the questions, how much last minute revision they do, and which way the wind is blowing. In a normal year, the exam would be the final arbiter. This year, you might decide that you can't, in all fairness, give them all As. That's where the internal assessments give you some sort of objective measure - particularly important because you know the kids personally and you don't want anyone accusing you of bias. What assessments schools used for that was up to them.
Teachers know full well that different assessment methods will give different scores: I don't think anyone would equate scores on each topic examined separately with scores on the whole syllabus. What was practical to organise will have varied between schools. Where a lot of curriculum time has been missed, they may have wanted to give students less stressful assessment, whilst realising that they need to bear that in mind in interpreting the scores.
I don't think anyone can guarantee that there was no inequity between schools, but hopefully the exam board moderation will have picked up the worst of it.