[VadeRetro]

The total energy decrease in the combined gravitational fields should equal the e=mc² equivalent of S.

I blew that part. Adding more mass adds more total gravitational energy. The increase outside beats the decrease inside.

[VadeRetro]

... total gravitational energy.

I confuse myself here with faulty terminology. I should say "gravitational field strength," as this is being equated to negative mass/energy.

[Physicist]

I blew that part. Adding more mass adds more total gravitational energy.

Actually, that sounded correct as written (unless I have a wrong idea about your geometry), because the gravitational energy is negative. If you double the gravitational energy, it's correct to say you are decreasing it.

The part that didn't sound right to me is where you said something about the field decreasing inside the shell.