Personally, I think that nothing needs to be modified in the marking because of Q1. I know that candidates will have been affected by different amounts, but maybe that is best sorted out by 'flagging up' the people who seemed to spend a substantial amount of time getting nowhere with Q1. The fact that some people were able to get past the omission and complete the question means that, in my opinion, there should be no change in the marking for this question. And this comes from someone who did spend 15 minutes and 1 page of writing getting nowhere with it. It would seem more appropriate to deal with this problem qualitatively (i.e. flag scripts up) rather than quantitatively (i.e. giving compensatory marks).
I do, however, fully respect what others are saying. I just hope, as a person who believes that they did really badly in II but reasonably well in III, that this does not put extra pressure on the grade achieved in II. If anything, getting a score of around 70 on III should be seen as an even greater achievement than normal, considering the difficulties with Q1.