Well I see it as this:
Minus Ellen, Gloria can only rely on the MC and Katie for protection, and once this is over, she'll only have Ellen to live with (if Ellen makes it through Book 2.) They do have a somewhat close sisterly bond, so it could be possible, but the MC/Katie safety route makes the most sense at this point, since like I said, the MC will always protect her and Katie, well, for being Katie. Plus, being a doctor is a big plus.
The MC will probably retire after this mission. He can set Katie up somewhere with a new office and they can stay together/get married or whatever, with or without Gloria.
In a perfect ending, that's what I would want for all three.