But you initially purchase kero and castor in plastic containers anyway so I am struggling to see your point about further degradation of those in said plastic containers.
(Does the ether combine with the kero and oil enough to be permeated along with it through plastic?)

IPN, well yeah.

For me there are too many variables (apart from the science experiment bit) to give any other answer than "when in doubt, chuck it out," and only premix what you intend to use straight away.

Thanks.