Angelo,
Not quite sure.
In the described scenario 1, boat A crosses the line from the course side. None of (a), (b), or (c) in the definition of Finish applies, as she only tacked which is not a penalty under 44.2, so (for the RC) she fullfilled the definition and finished, and could (should) be protested by RC
In scenario 2, if boat A tacks and gybes,  then doesn't cross the line, ok, she didn't finish (as her hull shall be completely on the course side before she finishes 44.2 last sentence). But I still think the RC committee should score her at her finishing order and protest (unless boat A is far ahead or far behind, RC has more to do than only deal with one boat). Finishing order is the main goal of RC (with all about fair sailing)
Loïc

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.

-------------------------
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?

Agree.  As long as it's getting our of RRoS ok.