But given unlimited time and resources there is no reason to assume that any part of Reality is intrinsically impossible to understand.
Fixed that for ya. Yet the universe is probably not unlimited, so no, one-time acts are not covered by science, and are yet observable. And they do exist, according to legend. The problem is that you'll never be able to "prove" they happened.
Actually, I fully think that the universe could be infinitely big and infinitely large. Given that, there is the possibility that no one event has ever been unique. I actually theorize that "light" has a lifetime, decay or attenuation... like what happens to your headlights on a dark night. Since there's less to get in the way in space we see tremendous distances and we could be seeing the point at which light can no longer propagate. In order to prove or disprove that we'd have to invent faster than light speed travel or a more accurate way to measure the distance to the furthest galaxies all around us. If we end up smack in the middle, my theory could be worthy of review. In a way, I guess this idea could be a possible support mechanism for your notion that things that can't be proven can also be true... but I'm not going to alter my life for such a theory.
I can't think of what you are referring to though. What acts of God do you know of that have ever only happened once? Literally everything recorded in the Bible has been explained (the great flood could have been a localized event [but it seemed like the world to those living there] like a great sea filling or something along those lines) or occurs on a regular basis (locust swarms, etc.)
For the record, the Bible is the only account of history I know that's been held up as truth in story (and legend, as you say... which is merely a fallible human account of an event.) That's the only reason I use events from that book. If you can think of another event, I'd be interested.