I don't think this is quite right.  It is making a RRS 2 protest look like a RRS 69 hearing which it is not.

I agree that a breach of RRS 2 that affects the fairness of the competition on the water will usually be accompanied by a breach of Part 2, but even if it is not, a breach of sportsmanship that affects the fairness of the competition is nevertheless an incident in the racing area and requires a hail of 'protest' and if necessary display of a flag.

Agree, and in OP case, I think we have

Boat B broke RRS 2.  RRS 36(b) does not exempt a RRS 2 breach.  On valid protest B should be DNE.

Couple of things from WS documents.

The WS Race Management Manual provides some guidance.
O.4 Finishing procedure
It is not expected of the race committee to follow the actions of every boat, especially in a race with a big number of boats. The finish vessel crew should record every boat that crosses the line from the course side. Possible double recordings of boats can be solved later.

The WS Race Management Policies Fleet Racing January 2024 provides some guidance in this area.
16. Race Committee Protests
16.1 Since the primary responsibility for protesting breaches of the rules rests with competitors, the race committee will not normally protest a boat.
16.2 The race committee may protest a boat in the following circumstances:
(a) A breach of a sailing instruction that may not be protested by another boat;
(b) An apparent breach of good sportsmanship (RRS 2);
(c) Failing to take a penalty after knowingly touching a mark, but not protesting another boat.

A lot of chat for a rather simple case.

To me... there are 2 halves to OPs question...

1st half - was Red racing?  Yes.  Her stern was still on the finish line when she broke rule 10.

2nd half - Did Red tack and gybe in accordance with 44.2...namely 'promptly' after the incident? This question is subject to the protest system, just like any other allegation of a breach of a rule not exonerated.

So the RC registers her second crossing i.a.w. Finish (a), and if anyone (RC or another boat) is unhappy about it, they can protest her for not exonerating 'promptly'.  Otherwise her 2nd crossing finish stands.

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As for the r23.1 discussion, Racing is a time-block from time-a to time-b.  Once you pass time-b it's over.  You can't restart.  If Red had met the interpretation of Case 127, then she was past time-b.  Not racing.

If a boat not racing breaks a rule, then apply RRS 60.5(c)

If a boat has broken a rule when not racing, her penalty shall apply to the race sailed nearest in time to the incident.

What am I missing?

Yes you can only postpone before the start. You can do it in the last second. 
You start when the flag is removed so, if this had a few seconds delay, the start would be late but there is the posability of postponment. 

Just follow the procedure and be practical. 

But, unless I am misreading the original post, the “restricted finish” in this case is 2/3 of the way up the course and nowhere near the starting area. The only boats playing in that area are the 18s and the H16s would only be there when finishing. So, again, I’ll ask, what issue are we trying to resolve by restricting this finish?