HL2 and MGE is a crossover I never thought I'd see, but sure I'm game.
On the topic of suppression fields and any anti-MGE measures, you're probably 100% on the money there. The ridiculous coincidences that makes every single aspect of the MGE setting relate to sex somehow leads me to strongly suspect that the DL is a reality bender on par with the Blind Idiot God. This is why those spacebattle numbskulls and their Dues Vulting makes no sense: you can't fight an opponent who can bend reality, casuality, and outcome to her whim. Even if you find a way to win, she'll just make it so you don't win.
Now, onto the part about negotiations with the combine. Simply put, there really isn't any kind of meaningful negotiation that the combine can do. In all likelihood, it'll play out with the DL giving the invading combine an absurdly good deal, dedicating huge amounts of resources to the combine order. Then, ten years down the line the advisers realize too late that 90% of their conquered planets and militarized forces have become sexy waifus who are far more interested in settling down and fucking than enslaving galaxies. And even if they try to mount an offensive, they all fail in comical ways and eventually become waifus themselves.
Do remember that the current DL is, Suzumiya-esque shenanigans aside, essentially a ferociously ambitious succubus. She overthrew monsters, she overthrew gods, she's currently in the process of effortlessly overthrowing humans, a multi-galactic alien empire? That's just another thing to absorb into her dream world.