My headcanon was that she has a Hydrostatic skeleton, like an Octopus.>>116856165
I'm thinking a mix of both. By disabling the safeties on the Omnitrix that ensure that Ben's original DNA is saved and restored between each use of the watch, Ben would be able to quickchange and have a lower recharge time.
The side effect wouldn't be apparent for thousands of uses, if not more.
The transformation into Omnibabe would be a slow one, occurring like that of taking a Xerox of a Xerox ad infinitum. Just a singular lost genetic memory of Ben's original form DNA sequence would warrant the watch filling in the blank with the DNA of everything to keep Ben alive. And it probably wouldn't warrant any difference at first.
But doing so over infinite time, Ben's genetic structure would be mutated and mutated over decades into what the watch THINKS a human is like, combined with the constant concubine reprogramming to warp Ben into something new. One day, Ben's bodysuit is now a skin. A year later, he's no longer beholden to human joints and elasticity.
Boobs and curves? Well, she's been female for decades, hasn't she?
The frailty of human existence falling away like Theseus's Ship to an infinite shore of time is a doozy. An incredibly sexy doozy.