At any rate, yes, Hiryuu survives. This is not to 'balance the odds' or 'buff Japan' or anything like that. Japan loses in the end anyway, we all know that.
No.
That was entirely for character reasons. Both for Hiryuu and Enterprise. You're already seeing some of that. Enterprise, in particular, is going to be changing quite a bit in this war...especially with the specter of her alternate self hanging over her.
Speaking of
Hiryuu, I suspect that she might assume that
Enterprise was the carrier that killed
Kaga. Considering the combination of wartime security ('Don't let the Japs know the status of Sister Sara') and IJN Naval Intelligence's
notorious track record on thinking USN CVs were sunk when they...were not (Yorktown twice, Enerprise and Saratoga more times than I can count), Tokyo probably will chalk off Saratoga as destroyed just like Kaga was.
Which means that Hiryuu will probably think of Enterprise as her rival/foe, assuming they got the memo that Hornet was still shaking down and Yorktown was unavailable. That leaves Little E as the culprit (and also has an eerie parallel callback to what happened between Enterprise and Hiryuu at Midway).
The thing I'm wondering about is how this will affect the Atlantic/Pacific debate between the Navy and Army. Will the Europe First stance of the Army/AAF (wherein they wanted as much manpower and material for Europe/Africa operations ASAP, even if it meant Pacific requirements were ignored IIRC) keep as strong a hold as historically, when the Navy has just given the US it's first major morale boost in the midst of the string of victories Japan is starting to accumulate?
...Also, would this victory potentially negate the need for a Doolittle Raid a few months from now?
Well, assuming that Hitler repeats his historical mistake and declares war on the US after Pearl Harbor, you will probably see the historical US strategy play out. The Pacific will be a war spearheaded by the Navy and Marines, just like Europe will be primarily the Army. While there will be fights over resources, the US economy will have enough for both theaters to be handled, even if not quite as much as some of the commanders would want.
Considering our primary POV characters are in the Kriegsmarine and then Enterprise, Sara, Arizona, and Thompson, the Pacific will not go wanting for ships and planes.
As for a Doolittle Raid, it is possible, although the morale should be somewhat better. Sara's pilots can confirm that the first installment for Pearl Harbor has been taken, and if any pictures were taken of
Kaga burning, I'll expect them to be shared with the newspapers ASAP. You do want to take steps to regain the initiative in the Pacific ASAP, and hitting Japan ala Doolittle is one way, although there are other potential targets in the Central and South Pacific available as well.
That said, there are dark days ahead since the Philippines are going to fall, and the Asiatic Fleet is in big trouble. I do expect that the next major carrier battle in the Pacific will be either Coral Sea or Wake Three, since clearly Wake is shaping up as effectively the AU equivalent to Midway as a locus for Japanese strategic obsessions.