The cut off is determined beforehand so that won't change. For eg if everyone in the state got 95% raw then everyone in the state would get a band 6; there's no limit on how many can get band 6.
Scaling is based on the overall strength of the cohort for that subject, so it depends on how everyone doing modern goes in their other exams.
The cut-offs are NOT determined beforehand.
The aligning panel will meet during or after the marking process and are markers as well.
There first meeting will involve a discussion about the difficulty students are experiencing with the section of the paper they are marking (and in Modern History at least one of the aligning committee will come from each section of the paper). They will then individually determine what they think the cut-offs should be for each question or part thereof e.g. one judge might think that the cut-off for Band 5/6 for Part II should be 21/25 while judge 2 might think 18/25 and the other four judges will also make their own individual recommendation. These recommendations are sent to the BOS and then in round two of judging they have access to some statistical information and re-do their recommendations. Round 3 sees them accesses samples that fit their cut-offs before their individual final recommendations are made.
For Modern History marking begins on Saturday so judging will begin sometime after that but probably no earlier than Saturday week so the judges have a week or more of marking behind them.
I have been on the Modern History judging panel in the past and we finish making our recommendations after marking has ended.
Scaling is done separately but also after the exam not beforehand.