This is the best list I could come up with of the events that transpired in Animorphs before #37.
As you can see there's a few things that I imagined Visser Three would have managed to have kept under the carpet, such as the David fiasco, the Area 91, the incident with the prototype Bug-fighter etc...
Also, I believe, the feudal relationship between the Vissers would have been kept under the blanket by both parties. Much how every Yeek knows it's there, no one puts it on facts for consequential fears of their leaders.
Please don't hesitate to let us know what you guys think!