The only way to avoid mistakes is to do nothing. But that's a mistake in itself!

Leaving the cap off will account for your oil contamination. The oil on the plugs will probably have come from the oil in the plug wells. Spark plugs run hot and burn off any oil that gets on them. What you see from oil burning is black tarry residue. If you had liquid oil it was from your oil contamination.

Relax, enjoy your car.

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Cheers, Neil