In general, we can never prove anything is ontology because systems can always be simulated in multiple ways. We only have access to perceptions, sensory experiences. We build a mental model of some external reality (the 'ontology') that explains how those perceptions arise, but the behind-the-scenes reality can look quite different to the perception. Appearances can be deceiving.
In the case of the measurement problem, we have a conflict between two different ontologies that seem to arise naturally from our perceptions. There is the 'wave' picture, where the wave passes through both slits at once, and the 'particle' picture, where the particle can be in only one place at a time. Our perceptions appear to 'prove' both of them at once. If the wave collapses to a particle flash at one end of the screen, somehow it knows not to do so at the other end of the screen, even if there is no time for any lightspeed signal to get there, and which in general leads to all sorts of weird backwards-in-time causal paradoxes in the ontology. We can just as easily say that the only-ever-observed-in-one-place-at-a-time behaviour 'proves' a particle ontology.
At least one of these appearances is deceptive. Either the ontology is a wave which somehow gives the appearance of only ever being in one place at a time, or the ontology is a particle which somehow gives the appearance of being in several places at once. We can't take either argument for granted.
Some people say that the particle picture is the deception, that is caused when the wave of the photon interacts with the wave of the observer, correlating them, so the observer becomes a superposition of orthogonal mutually-non-interacting states, in each of which the photon is only seen in one place. Some people say that both proposed ontologies lead to intuitively unacceptable conclusions, both proposed ontologies could be deceptive appearances of some other ontology we haven't yet thought of, we can't possibly find out by any conceivable experiment, and so there's no point in wasting time speculating. It's not a scientific question.
To believe we have answered the question of ontology, we need an explanation that explains everything about what we have observed. If it's a wave, how and why do we only ever see a single flash in a single location? By what mechanism does one part of the screen know not to flash because another part is about to? You can't say this part proves it's a wave, and then just wave away the parts that 'prove' it's a particle. Nobody has yet managed to produce an explanation that fills in all the details to everybody's satisfaction, so the debate goes on.