Minor points:

The RC MAY abandon, does not HAVE to.

A protest committee, in hearing a request for redress, or as a result of a protest hearing from one boat vs. another, MAY abandon the race. They could also decide a different arrangement would be appropriate.

Hypothetically speaking... Suppose the RC did set this up intentionally, and had posted the SI change on the board; even if only one boat went around the inner mark, it would be difficult for the RC or the protest committee to justify abandoning the race, or even granting redress. The error would have been committed by the sailors who failed to check the board.

Fortunately, the sailors actually came up with the best answer in this case. So long as they all went around the same weather mark, regardless of the string rule on the inner mark, no one was at a disadvantage and the race was saved, probably the only way both of these results could have been achieved in that case. If the leaders went around the inner mark, and all others followed, I would argue that would have been an equally acceptable solution.

The SIs aren't the best, but it sounds like this was compounded by two RC errors on the water. The RC owes the fleet some beer over this one, as the sailors saved the race.

If the RC did this intentionally, but failed to communicate this to the sailors as an SI change on the official notice board (skippers meeting would be a courtesey), that would also be an error.

Mike