For your Question “How are you convinced that there’s any co-creators outside of/besides you? What if you’re the only being creating? What makes you so sure there’s any souls outside your soul? This goes for the world/cosmos too, you don’t know if there’s an external outside reality independent of your soul/consciousness. So, how can I take comfort in knowing for certainty that I’m not 100% alone, at the most cosmic scale/level”.
All observers/all actors are ultimately one. The “you” and “I” are relative in the world of experience of space-time. Ultimately, there is only One. We are not promoting solipsism, we are saying that the world as utlimately being separate from underlying conscious Awareness does not exist. But as experience, in space-time, it does exist. This is the point of complementarity.