Certainly there could be other plausible explanations.

What do you mean by deletion and inversion? Are you talking about the nature of the lesser mutation it’s self rather than changes in other genes? Lesser could certainly be a different type of mutation (deletion or substitution) than other compatible mutations (Mojave, phantom) of the same gene but for it to vary within the lesser line to create some platys are you saying that the lesser mutation is mutating again to create platys?

Sure the sample size isn't huge but the lack of platys so far without inbreeding the lesser line makes me think homozygous for a 2nd gene. The tricky part is why that 2nd gene doesn't have a reported effect in all the breedings of normal looking platy offspring together. I think the rat dilution gene gives a good precedence for a plausible explanation. Of course it will take years to really prove anything but again this year should add some good data to help figure it out.