Here's you've just shown that you have no idea what Aubrey and SENS are proposing. Maybe, like with Hofstadter's book, you've just "read the introduction" and then stopped?

>We have no idea which genes contribute to aging, and how, and even how many. In all likelihood a simply astronomical number of genes would have to be altered to extend life.

The whole point of SENS is that it DOESN'T try to fiddle with metabolism (our genes). All of the genetic diseases you mention are different from the diseases of aging, so they're a bad example.

SENS is all about maintenance and periodical repair (like with an antique car, for example). Every, say, 10 years you go in and they remove accumulated damage (which can only be present in a limited number of long-lived molecules) BEFORE it causes pathologies. Most of this can probably be done with your own immune system (you train it to recognize some molecules), and some of it will have to be done other ways (gene therapy to make your body produce certain enzymes, etc).