Hehe, thanks man, but sadly, Dead Island 2 was never shipped so far 
Regarding some of your statements, I have to strongly disagree^^ GI doesnt only make things brighter, GI also occludes, thats the logic of light bouncing around, because if it cant go somewhere, it becomes automatically dark there. Thats why Lightmass, which is a GI solver, makes such nice dark corners below stairs for example.
What I am trying to say is, if you have a good GI solver, you dont need anything else because everything that is not direct lighting is GI. This includes things like shadows and AO as well if you take it from a scientific standpoint. However, that doesnt matter too much for us. But again, if you bake Lightmass, you will get dark interiors and that is because the global illumination doesnt bounce around in the building as much as it does outdoors. So any “good” GI package will give you proper sky occlusion since its just a different aspect of the same thing. Funny fact though, most people refer to GI only in the sense of local color bleeding, which makes sense, but still doesnt reflect the full picture
Regarding LPVs only making things brighter and not darker…as far as I know (but I might be wrong) that is outdated information. I believe I read in the Docs and Update CLs that LPVs support injecting the skylight and shadowing it based on the LPV data. So to my kowledge that should work^^
On the other side, regarding DFAO…you are right that the occlusion distance is limited. However, It still does a fairly good job in my opinion and if you dont build roman domes, you should still be kinda fine I guess? 
Well…in any way, some occlusion is better than nothing, so I think it can be worth a try^^
Cheers! 