The biggest issue here is to try to determine - where - did an entire Gallon of milk go, and then go there and try to remove all of it if possible...
I am sure the entire gallon went to the front first since it was on the front seat and then ran back all the way to the floor under that side of the back seat if there is a hump all the way back to keep it on one side...
And then you have all the carpeting and pad underneath that got wet with milk, so every square inch has to be looked at and cleaned, extracted a lot, and cleaned again to be sure you did not miss anything...
Once the front seat and back seat are out, you can look at the floor and probably see the trail of it and better still follow the smell and eliminate it...
I agree there may be deep places in spots - Im thinking that the floor sill might also be a place since it is usually a tunnel, where some wiring is in, may also have some stuff in there so you need to check there since you will already have the carpeting out, right ?
I think I have used Bio Power before on a BMW that had been left outside in a rainstorm for a week with the windows down... Man that thing had several inches of water in it...
Thinking about that nightmare, I remember I had to use my Mytee HP60 extractor with all 3 motors winding up as high as they could go, to pull all the water that the bottoms of the foam seat cushions had absorbed... It was a huge amount and this type of BMW foam was very, very, uncooperative about releasing the water to my extractor...
You might check the foam of the seats - with the sniff test - and see if they are holding some milk in the foam as well...
You are doing an awesome job, Shane; your work has always been stellar...
Just keep moving forward, focus on an area, get it totally done, then move on to the next area...
Coming from the military, I always liked to grid things out so I can see it and then get everything in each part of the grid done before moving on, so I dont lose my place..
A way less reputable place would have soaped it up, perhaps dried it using the car's heater and a/c units, the put some of those nasty toxic bombs in there to mask the lingering odor long enough to get the car done and long gone...
We are all so glad you are not those guys !

Good Luck - You Got This -
Dan F