One mutation being enough to create "a functional prototype of a complex feature/organ (MVP, if you please)"? That's not how reality works!

You can't introduce a complex organ out of the blue by flipping a single gene anymore than you can add HTTPS handling to your webapp by changing a single opcode in your binary. For that to work, the structural and operational information must have already been there in the organism.

I think that's exactly it. The groundwork for "complex" features must already be there in some sense, maybe through mutations or hitchhiking. The last mutation is probably like flipping the on-off boolean. Or maybe something dramatic like a keystone. I.e. the parts alone don't do anything, but you have this magical bit flipped that suddenly turns everything on and gives them significance.

But yeah one bit/mutation does not a feature make. Not without some prior stuff to latch on to, in my opinion.

An injection system is not an 'MVP' of a flagellum, an organ for propulsion. What doesn't sound right is the idea that single mutations often produce essentially working versions of future complex features.