Yeah, pretty much the dream machine is a MacGuffin it doesn't do anything but allow the plot to be possible. It's something you have to hand-wave away as how it works isn't the point of the story. Hell even the heist
isn't part of the story. They both just are vehicles to allow them to tell the real
story. Namely our perceptions of reality and more so Cobb's own internal struggles with the death of his wife and his involvement with it.
He "blames himself" for her death and he's trying to cope with that and recover from that so he can move on with his life and be with his kids again. That's what this movie is about (and why the ending is in the "real world.")
People who have lost spouses likely "blame themselves" for either not seeing the signs or not doing enough to prevent it or maybe even doing or saying something that may have caused it. (When in reality the suicide was caused by the person who committed it and no one else.) So Cobb's suffering from the death of his wife sent him on a downward spiral that made him an addict to substance abuse (in the movie the dream-machine and dream-sharing fills this role), detached from his kids (in the movie by being thousands of miles away from them for an extended time). Over the course of this movie Cobb confronts these inner demons, rejoins his life and stops blaming himself.
The dream stuff is just an interesting way to present all of this to an audience.
Now, on the DVD set, as well as on the internet, there is a a "copy" of the dream sharing machine's manual but it's more on operations and how to use it. It doesn't tell you HOW it works sort of like how your TV's manual doesn't get into rapidly electrifying pixels into a desired pattern.
But we can infer that the dream machine cycles the drugs across the bodies carrying the "dream information" with some component of the drug. Somehow it's able to do this without blood type needing to be an issue.
It's possible better thought needed to go into presenting the device (by needing them to put something in our around their head) to give things a better sense of the brain involvement but, again, how the dream machine works doesn't matter.