Popeye was originally a walk on character in a comic strip called Thimble Theater, so I doubt Segar created the character for the express purpose of advertising spinache. Also I should note that although spinache was mentioned as the source of Popeye’s strength in the comic strip, it wasn’t nearly as important as it was in the cartoon series.
I’ve only got a WAG as far as the Olive Oyl question is concerned, Quasi. If what this page says is correct then the name was a detriment in places like Europe rather than an asset to sales of the oil.