Putting up the M flag and repeating horns is the correct thing to do (you should have done this on the first rounding, but you were busy trying to save the mark).
A semantic correction - a competitor cannot "protest" the RC. They can "request redress" which is procedurally similar, but has some specific conditions.
RRS 62:
Quote:
62 REDRESS
62.1 A request for redress or a protest committee’s decision to consider
redress shall be based on a claim or possibility that a boat’s score in a
race or series has, through no fault of her own, been made significantly
worse by:
(a) an improper action or omission of the race committee, protest
committee or organizing authority;
(b) injury or physical damage because of the action of a boat that
was breaking a rule of Part 2 or of a vessel not racing that was
required to keep clear;
(c) giving help (except to herself or her crew) in compliance with
rule 1.1; or
(d) a boat against which a penalty has been imposed under rule 2 or
disciplinary action has been taken under rule 69.1(b).
62.2 The request shall be made in writing within the time limit of rule
61.3 or within two hours of the relevant incident, whichever is later.
The protest committee shall extend the time if there is good reason to
do so. No red flag is required.
I bolded the important phrases applicable in your case.
The first question a Protest Committee will ask will be, "Was the boat's score made
significantly worse through no fault of her own?".
In your case, the fault is certainly not the competitors', however, was any competitor's score made significantly worse? From your description, it sounds like everybody figured it out and there was no
significant changes in position.
If the Protest Committee determines that there is a legitimate cause for redress, the next question will be "What will be the fairest arrangement as possible for all boats affected, whether or not they asked for redress?" (Rule 64.2). In this case, it would be difficult to determine a fair result for everybody, so they may decide to abandon the race (throw it out).
So the short answer is - you did all you could. If someone requested redress, then I give it a 50/50 chance with the protest committee.
Oh, and I agree with Rick - send the H-20 driver a bill for the mark and the ground tackle.