You can actually kind of fit three windows on an iPad: two in Split View and another one on top in Slide Over. Even if you can’t interact with the background windows when Slide Over is active, they continue to refresh their contents.
Fun fact: I believe you used to be able to interact with the split viewed apps in early betas of iOS, but then the feature was removed and restricted to only when you had one app open.
You can interact with two Split View apps and one Slide Over app at the same time, but this is limited to newer models (I believe 2017 iPad Pro or newer). If you are on an older iPad, it is as you describe.
Yes, but this used to be enabled for all iPads. The feature was then removed, probably because older ones just couldn't keep all the apps running reliably.