A fair question, but I would say that everything we know about black holes, everything, comes from extrapolating the equations of physics as found through experiments on this side of the event horizon. That is the natural logic-- we do experiments here, develop theories, the theories make predictions, and the predictions have their own consequences here. If in the process, we create an image of something on the "other side" that helps us organize the data over here, that's fine-- but we don't need to say that our picture of what is going on inside a black hole is real, beyond its impact over here.

I would use the example of "image charges" here. We know that if you put a charge near a conductor, the conductor will impose a boundary condition that lets you figure out the electric field near the charge. This you can determine without knowing what is actually happening inside the conductor, and indeed you can model it by replacing the conductor with an "image charge" that serves to create the same boundary condition. If you never do observations inside the conductor, the image charge approach is a fully successful model of the reality where you do the experiments, and your scientists would never be the wiser that conductors do anything different. If you do experiments inside the conductor, you find it is not a sufficient model. But with black holes, the latter never happens, so we need not distinguish what is "really happening" inside a black hole from the image it leaves on the outside.

The connection to multiverses is that there we have the worst of both worlds-- we never do the observations on the "other side", but they also don't establish any conditions that are relevant here. They don't help us calculate or predict anything happening here, they don't put any constraints on the physics here. They are purely a philosophical tool to give a sense that there is not a mystery where there in fact is a mystery. It's classic magical thinking-- the inclusion of an untestable mechanism to replace our discomfort and powerlessness about mystery. That's a valid approach for some things that humans do, but science was expressly designed to get us away from that-- how ironic the ways it tries to sneak back in, like a wolf in sheep's clothing.