Reference
http://www.englisch-hilfen.de/en/grammar/if.htm
http://www.englisch-hilfen.de/en/grammar/conditional_special.htm
https://grammarianism.wordpress.com/2015/05/10/determiners/
http://www.verbix.com/webverbix/English/say.html
You are correct, this is the "unreal condition", per the above reference, type iii condition not possible to fulfill (too late). Normal structure "would + have + past participle"
However, the complication comes in that the conditional sentence also contains a relative determiner (what) and a pronoun (she) and implies completion (she have said), subjunctive perfect tense as opposed to the past participle. Subjunctive perfect indicating possibility as opposed to indicative perfect representing certainty.
Look at the progression from the classic rule, to introduction of determiners and a pronoun:
- if the circumstances were different, I would have said something else
- if the circumstances were different, what would I say (use relative determiner)
- if the circumstances were different, what would have occurred (type iii classic with determiner)
- if the circumstances were different, what would I have said (subjunctive perfect)
- if the circumstances were different, what would she say (use pronoun)
- if the circumstances were different, what would she have been saying (continuous perfect)
- if the circumstances were different, what would she have said (subjunctive perfect)
Therefore the sentence reads, if the circumstances were different (if + simple past), what (the relative determiner) would she have said (unreal conditional + perfect), meaning, "what would she" (lady Catherine was it?) have said. The tense is subjunctive perfect because the action would have been completed, but not with any certainty.
Additionally, indicative perfect does not work here.
- WRONG: if the circumstances were different, what would she has said
That sounds horrible, it just doesn't work because you are speculating (conditional) whilst saying that "she has said" (indicative perfect) something which is a contradiction.
"If he had chosen differently, how would he have turned out?"
Yes. This sentence reads well and implies, if he had chosen differently, then speculatively, what could possibly have occurred, what might his job have been.