Question 1
Two boats A and X are on a beat, having a close tacking duel. Both are on starboard tack and A is subject to rule 17.1. A luffs to tack astern of X, which she can do if X does not change course. As soon as A starts to luff, X also luffs.
A passes through head to wind and makes contact with X's quarter. A protests.
What should the call be?

Answer 1
Until A passes head to wind, X's only obligation is to keep clear. A is subject to rule 17.1 and may only sail above her proper course if she promptly sails astern of X.
As soon as A passes head to wind, X becomes right of way boat and any change of course from this moment must comply with rules 16.1 and 16.2. If contact is caused by X's change of course after position 2, X breaks rules 16.1 and 16.2. Penalize X.
If contact would occur even if X holds her course between position 2 and 3, A breaks rules 17.1 and 13. Penalize A.

Question 2
Will the answer be different if the contact occurs before A reaches head to wind?

Answer 2
A breaks rule 17.1.
In addition, if X is able to respond to the change of course by A, but fails to do so and does not keep clear, then she breaks rule 11. Penalize both boats.
However, if A's alteration of course does not give X room to keep clear, then A also breaks rule 16.1. Penalize A only.