Each school, to some extent, can make their own decision as to how they calculate their students' grades.
Last year again each school could devise their own way to ascertain their CAGS but they also had to rank the kids and in our school this is where the mocks were helpful (but not set in stone). The pupils were also told that it was unlikely that their CAG would be lower than the grade they achieved at mocks.
As regards the mini assessments set by the exam boards we have opted in as they will be happy with the difficulty of the paper and set the mark scheme. If a school sets a paper and the board requires evidence to back up it grades there may be a risk that the board says the paper is too easy and therefore the highest grade for it can't reach an A*. Hopefully no school would do that.
In a selective/super selective it is very difficult to separate some of the A*s from As as can be seen by prior A level results based on all the factors that an exam bring into it. By using the board assessments it brings some of those factors back in albeit they will not be sat in large halls etc.
With the mini assessments remember that the school selects the topics for them and will be allowed to tell the pupils what topic is covered. So it will not be the same as revising for all the course for all 3 or 4 A levels or PreUs.
Thus they will be more like end of topic tests.
Other than CIE who have already indicated to a large extent what they want (but are obviously not bound by JCQ rules) the full information has not yet been received by the schools and we are still expecting it by the end of the term (before Easter) so they will know more then.
However for anyone taking CAIE then A level results will be 12 August not 10th and Pre U results will be 19th August so who knows what effect that will have on UCAS track updates with uni decisions too?!
CIE have been quite specific about what they require for their A levels and PreUs.