Basically, all that the ISAF clarification is saying is that that if the SI's specifiy a gate, then RC must set a gate. This is their way of recommending that the SI's provide an alternative rounding if only one mark is present.

This falls under the category of "improper action or omission of race committee", and could be grounds for redress. If the SI's specify a gate (and no other option) and one side is missing, then boats could reasonably assume that:
1) This is not the mark - in which case they may go on sailing looking for a gate, or
2) That it is the left gate, and round it to port, or
3) That it is the right gate, and round it to starboard.

If a boat boat decides that it is not a mark, then rule 18 would not apply.
If two boats decide to round it the same way, then rule 18 would apply between them.
If two boats decided to round it on opposite sides, then rule 18 would not apply and things can get hairy very quickly.

The recommendation for Race Committee: Don't do that. If the Sailing Instructions call for a gate (with no other option), then you must set a gate. If one mark disappears, then you must reset or replace it (possibly with a boat flying flag "M"). If you don't, then expect to either abandon the race or spend some time in a redress hearing (or both).

I hope that helps,
Eric
US Sailing Certified Judge
Area D Appeals Committee