Re: "This means that putting up a non-neotenous front is not even slightly incompatible with an actually-neotenous nature." Okay, so what would it look like for this to even be a falsifiable hypothesis?

Oh, to be clear, none of this is even slightly close to being falsifiable at its current level of development.  We’re still in Telling Interesting Campfire Stories territory here. 

(I imagine that you could design experiments where you looked at relatively “uncivilized” peoples, to see how the behavior of their actual adults was different from the behavior of their kids-trying-to-act-adult, and then did some kind of comparison to different populations of more-“civilized” folk.  But that is spitballing.)

The point is really just to forestall all the people who are obviously going to say (or at least think) “yes, neoteny is on the rise, it’s terrible, and the solution is definitely that culture should encourage people to be more like Cool Hard-Edged Adults such as myself.”  Having a counter-neotenous aesthetic or morality doesn’t intrinsically make you any less childish, in a way that a four-year-old wearing his dad’s tie is not thereby rendered more genuinely adult in any way.  That is a thing that would have to be established somehow.