If we were able to grab Manila the threat of a massacre should be limited by the fact that general Yamashita will have control of his troops, unlike in OTL when he was cut off in northern Luzon. We held him responsible and executed him anyway...an injustice as far as i can see. Iwo and Okinawa may not be as tough because they won't have as much time to build the defenses. The Mandates would be the same as OTL, I suppose. We end up with the same problem, though as has been brought up before...we get to the home islands "too soon".
Given that the Europe First policybis still in effect, the British are in a much stronger position ITTL, and the Germans have been doing better in Russia ITTL, after this victory, it's quite possible that while Nimitz can and will get more and more forces to fight with, a number of units, especially Army and Army Air Force units that historically ended up in the Pacific might instead be redeployed to European duty.
Even after this victory, the Allies still lost a lot of ships and aircraft, and thousands of families are going to have terrible letters in the mail, or service personnel making unfortunate visits. A lot of the new ships coming online are going to need time to work up and train as well.
If nothing else, by Q3 of 1943, I'd expect Nimitz to clear the Mandates and move to take the Marianas. The former might even be doable by May or June.
Another thing to consider, Japan at this point basically has a military government. You have the Emperor and some civilians in support roles or powerless administrative or figurehead posts, but the senior officers of the Army and Navy are running things for the most part. Whereas Nimitz, Somerville, Eisenhower, and other US or British commanders still report to and serve at the pleasure of their civilian masters, in the case of many senior 3 and 4 star Japanese officers, with the exception of the Emperor himself, there is nobody over them. The reason I bring this up, aside from the number of ships lost, quite a few of these ships likely went down taking one or more Admirals with them. Nagumo was mentioned as being fatally wounded. He also had a 2 star Rear Admiral serving as his Chief of Staff. Did Yamaguchi survive the loss of Hiryu, or did he go down with the ship. Yamamoto may or may not survive, and I'd assume he would have either a Rear Admiral or maybe even a Vice Admiral serving as his Chief of Staff aboard Yamato. Given the number of other battleships, battlecruisers, and cruisers that have been sunk, plus the number of damaged ships, it's probably safe to say at least two or three additional 2 or 3 star admirals have also likely been killed. This is going to create a major shakeup at the leadership level as well. After Midway OTL, with the exception of RADM Yamaguchi, all the senior officers that took part in the Midway operation survived and went on with the war. Here, numerous senior leaders are not going to be present in the aftermath, and numerous flag level commands don't exist anymore. The Japanese aren't complete idiots, many are actually western educated, and they just got their asses kicked hard. I don't see them surrendering anytime soon, but I could see their navy at least becoming somewhat willing to play things a bit more on the conservative side.