I was going to question the priming too. It is always advisable to prime store-bought "pre-primed" canvas yourself. They use the cheapest canvas and the thinnest "gesso" possible.
If you are confident the Walnut oil is good (it can go rancid easily but I think that's more of a smell issue) and the paint was dry to the touch, it must be the priming. Some artists use the "fingernail test" to check for adequate curing before glazing/topcoating (jamming your thumbnail into the paint to see if it dents) but you say the underpainting is very thin so that seems irrelevant.
A spray on varnish could be used to protect the underpainting but that would go against your solvent free approach (spraying varnish is not very eco-friendly!)
