I still have no idea of the circle/single oval/double thing you mentioned.
Imagine a bicycle chain layed flat so the pins connecting the links are horiziontal and you are looking straight down. This is a single chain.
Now imagine two bicycle chains layed tightly side by side with both inside connecting pin ends touching each other and horizontal and you are again looking straight down. This is a double chain.
You can see that the double row timing chain, like two bicycle chains, is about twice as wide as the single timing chain. And the double row timing chain has about twice the strength of the single chain.
Chain stretch was mentioned by others. This is not technically accurate. The pins and the rollers wear, oval on the inside of the rollers and flat spot on the pins, so the chain becomes elongated but not stretched like a rubber band.
Another thing, the only time the chain becomes loose is when it elongates beyond the capacity of the chain tensioner to keep the chain tight. A competent MB mechanic can inspect the timing chain and tell you if it needs replacement. He should also inspect the chain tensioner for proper operation.
P E H