I’m a pacific war historian, so I’d only be able to give you the perspective from that side of the conflict.

For the early part of the war, the US was on the back foot and defeats and draws were the norm minus a couple of other events such as the Doolittle raid. The media at the time was under wartime censorship so a lot of things were not going to be openly broadcasted. First off, the media needed to get hold of stories but these were not always easy to gather. The US Navy was notorious for their secrecy and ignoring of reporters. This dates back to the Spanish American war, in which the press played a huge part in that conflict to what many believed to be at a disadvantage to the navy. Young officers such as King and Nimitz payed close attention and came to the conclusion that the media in general would make everything more complicated.

The US army was somewhat different but had a similar philosophy. But in 1941- 1942, a major story was hitting the rounds and that was the Us Army at Bataan. They were outnumbered and going to lose and the media knew this but tried to rectify a defeatist mentality. MacArthur was proclaimed a hero over night because of this conflict and the media’s portrayals over it. He was given degrees from university’s, was made a tribal chief of several Native American tribes. A new hairstyle developed which bore his name and a new kind of dance also became popular named after him. This clashes with the navies idea of not getting the spotlight and throughout the entire war, it would be MacArthur and army VS Nimitz/ King and the navy. Both sides wanted full control over the entire theater, MacArthur had the media on his side and almost got it.

As you can see, the defeat was not the point of the publication and it somewhat took a backseat to other affairs. Yes the defeat was bad but the goal post somewhat shifted to “why was this a defeat” and “what can we do to improve” this is one example though from Bataan. Other defeats like Vella Lavella, the Santa Cruz islands and as you said, Savo island, and so on were not really a spotlight story. Partly because of the US navy but also because the media did not want to discourage the population. To answer your question, how many knew about Savo island, probably the entire nation later on in the conflict but they learned after it happened and at that point Guadalcanal was probably already won and the next story was taking hold.

I know your question was about defeats. But the outrage was usually directed towards victories believe it or not. The most famous one being Tarawa. This battle was heavily publicized and well known and the media put out some pictures and video of it. Bodies of dead marines washed up on shore and destroyed Higgins boats were all that was needed to make the American people outraged ( the media actually did not post the more disgusting and disturbing images of bodies mangled, bloated and bleached white from the sun, which veterans thought was interesting )

The Americans were angry at Nimitz, King and all the other navy commanders who allowed this operation to occur. The US Navy’s disaster of plans were also known. The reef and the mistiming of the support aircraft. The Americans were so outraged that one wrote directly to Nimitz and blamed her sons death on him. There were cries for MacArthur to take over the entire theater as he criticized the navy commanders for their faults in leading up to the conflict.

In summery, it depends on what defeat we are talking about. The US navy ones were not really publicized until later on which would lead to a delayed reaction and the US armies early defeats were turned into “how can we improve” and a renewed sense of patriotism and revenge. Even in 1944 - 1945 in Bastogne in the western front, it had a same mindset. Instead of being outraged that the troops were encircled, the American people were cheering them on. Same with Guadalcanal after Savo Island. The later battles such as Tarawa and Iwo Jima bore much more outrage by the American people who were able to see pictures and videos. Lastly, I’d like to mention that some battles would be outrages such as Okinawa in which 2,000 marines were killed or wounded in the sugar loaf area that is about a couple hundred yards long. Yet this story was buried between the German surrender and Roosevelt’s death, thus not getting the spotlight.