So you don't think that making Kaworu's creation work within an event that's already part of the show, and was deliberately added to the show after the fact (i.e., the Adam CE), is more parsimonious than requiring an entire additional event that the show provides no direct evidence ever actually happened?
It also doesn't make a whole lot of sense for Kaworu to be based on the donor unless, well, he was a direct product of the procedure the donor was involved in (on what happened to be the donor's last day on planet Earth). Why in the world weave an additional tale about how this guy's DNA is so singularly useful for weird experiments that, post-2I, his DNA needs to be dug out of some hairbrush from Japan in order to create Kaworu? If Kaworu were created later, they could have based him on anyone -- even, say, a NON-Japanese person!
Also, as I said, if the genetics experiment does not explain Kaworu, then you have to come up with a totally different explanation for why it's in the show, which takes us yet further from this idea of "minimal".
And I thought you were a big manga fan, so I'm surprised you don't find Sadamoto laying things out in the open more persuasive.