If you glue 4 triangular pieces together you should get the look you want. I don't think the corners would be prone to dings with the design you are...
I would guess based on your location that the saw spent considerable time in a shipping container which was probably pretty hot for much of the trip....
"Rainbow" poplar is just regular yellow/Tulip poplar that is stained from minerals in the soil it was growing in. There is no way to fully assure you...