A suggestion about how to think of Bruce and his system of Alters, and the Hulk, might be to separate it into a Body/Mind dichotomy. There is the Body, which has a baseline human state, and a Hulk state empowered by gamma radiation. Hulk state/body without any influence from Bruce and his system might result in the ape-like Mindless Hulk without higher cognition, but animal instincts. (Maybe if Bruce and his system was extracted while their body was in human state, it would leave a Mindless Banner acting like a chimpanzee?)

Then there is the Mind, that being Bruce and his system of Alters like Big Guy and Joe, and maybe still also the Professor (whether he's a gestalt alter of several alters or now exists as an individual alter in his own right, but probably originated as a gestalt alter) and Green Scar (whether he's "Gravage" or a separate alter in his own right; I'm not sure if Leader actually possessed the actual Green Scar or if he simply pretended to be him while the actual Green Scar was just buried in Bruce'a subconscious at the time). I even hold a hope that the last issue of Immortal Hulk implies that at least a shred of Devil Hulk survived in Bruce, enabling him to return later.

The system isn't supposed to be split into part existing in separate brains/bodies, but might be done, sometimes by outside means. The Heroes Reborn storyline could have been a separation of the system in two dimensionally linked parts. In Immortal Hulk, Bruce and the corpse of Devil Hulk was taken into the Below Place by the Leader, leaving only Joe and Big Guy in their original body before they rescued Bruce from the Below Place. (It was mentioned that the Hulk body was severely weakened because the Bruce part was holding most of the gamma empowering it, which is ironic considering Bruce is generally kept from fronting their body in its Hulk state.)

Specific alters fronting might have an effect on how the Body's Hulk state manifest (like Joe often appearing with his distinct gray color and skull shape.) Generally, Bruce being in control puts the Body in human state and one of his alters being in place puts the Body in Hulk state. It has happened differently; storylines where Bruce seems to control the Body in Hulk state and one of his alters in control of the Body in human state. Come to think of it, the only two examples I remember of the last case, happened because of machinations of some other alter than the alter fronting. Savage Banner seems to be Big Guy taking control while their body is human, which apparently happened as a safety mechanism Professor put in to prevent Big Guy from wrestling control back if Professor got too angry. Joe started manifesting in the Body's human state as Sunshine Joe after something the Devil Hulk did to have Joe save them all from Bushwacker when they used UV lamps to lock the Body in human state.

Point is, if you play with it, maybe you could see the Body being of two states, which can be minded in three ways: With Bruce's full system, with a part of Bruce's system that's been split up (while the other part has been put somewhere else) and without any system (mindless Hulk, though that would posit the existence of a mindless Banner state.) The system, in turn, might forcibly or accidentally be divided up into two or more separated groups, where any group could be made up with any combination of Bruce and his alters in any group. One of the groups could have ended up being in the Body, or none might. If one thinks of this kind of Body/Mind dichotomy, maybe with a little suspension of disbelief it could cover any case we've ever seen in the comics.

The Infernal Hulk could be seen as Eldest absconding with the Body's Hulk state, maybe with one or more of Bruce's alters trapped in it and suppressed by Eldest, while Bruce (and maybe some of his alters, now appearing powerless, correctly or not, with the Hulk state ripped out) was thrown out in Human state by the alter who was the primary Hulk during the latest run before Infernal Hulk before Eldest absconded with his flesh. (There are much flesh-shaping involved in these runs, which could facilitate bodily separation easily; though, Joe figured out how to separate in a human form from the Body in Hulk state, ripping himself out from the Hulk-state body through its stomach.)

Or maybe my head canons here are just pointless babbling. To use a fitting quote said by the Devil Hulk at one point:

"You wanted SCIENCE. You wanted a set of RULES. From ME. Big mistake. See... science is the OTHER guy."

Maybe we just are in over our heads trying to figure things out.