Im not understating anything, and I'm honestly not sure if you read what I said as I wrote it (unless I descibed my point of view that poorly).
I made a proposed modification but wasn't explicit enough about it and you understandably missed it.
My idea is that eevee would have a base set of moves. Such as tackle, scratch, tail whip, and growl. And then as he evolves, into one of 8 (or more in future) options, the effects and visuals of those moves would change.
The idea is that you could get a feel for your opponent with Eevee, and then evolve it based on what suits your match best.
My point is that Eevee would be terrible to play as. Cute, but plainer than Mario. There isn't even as much to do with choreography given the quadrupedal part, there's just nothing there.
So I suggested skipping Eevee entirely and having it be a choice between some of the evolutions for each stock, but the problem of how much is recycled is still unclear (and it would be even if you reject my earlier suggestion).
If you want to properly represent the better ones well, then there can't be that much overlap at all.
What that leaves is having effectively multiple full characters in a single slot so that you can counterpick in-match, which is in danger of being completely unbalanced 1v1 because it means that you can basically have a decent-to-positive matchup no matter who you're against.
And if that's not what that means, then the premise isn't actually being executed on, unless they're all underpowered, and the matchup advantage is just mitigating that.
Never mind, I guess I answered my own interrogation. I don't really like how it forces you to pick the better matchup, but that's partly because I'd be partial to playing one of them all the time.
So my only surviving objection to your idea is that playing as Eevee seems terrible, even if it's only temporary.