Unless I'm not understanding what you're saying, it sounds like you're just describing two combos that share some actions, like WAR's Storm's Eye / Storm's Path combo or literally all of NIN's combos. That's easy. I get how that would work. You'd have as many combo buttons as you do combo finishers. Easy peasy. The difficult puzzle to solve is specifically the two situations mentioned in the parenthesis; MNK using Perfect Balance, and SAM using Meikyo Shisui. Those two skills that allow you to press any of your combo actions in any order at any time for 10 seconds. This necessitates that all of your actions be available to press as the situation demands. I think SAM is solvable. MNK, I'm not so sure.
I've been leveling GNB, so I'm well acquainted with your example. I do that too. With that said, I'm failing to see how changing combos from 1-2-3 to 1-1-1 would in any way alter how your example is played. You'd hit 1 once to do Keen Edge. You'd hit 1 a second time to do Brutal Shell. You'd then hit other buttons just like you already do, and come back to hit 1 for a third time to execute Solid Barrel when you're ready for it. The only thing the chain-combo button would prevent you from doing would be resetting your combo early, like if you wanted to spam Keen > Brutal to spam the defensive buff.
It's examples like this that make me think that some people just don't understand the chain-combo button suggestion. This in no way would alter how you would play your jobs. We are not asking for a single button press to execute the entire combo chain. We are simply asking for a single button that changes what action it executes depending on where you are in the combo. You would still be pressing the same number of buttons, you would simply need less hotbar space for all of it. Is THIS the misconception that keeps making people respond with "but that makes the job too easy"? You know how WAR's gauge spenders used to change based on what stance you were in? You know how we didn't have separate buttons for Inner Beast / Fell Cleave? That's literally all we're asking for, except applied to combos. This change does not reduce the quantity of button presses.
You press 1, you execute ONLY Keen Edge. Now the action displayed on 1 changes to Brutal Shell. You press 1 again, you execute ONLY Brutal Shell. Now the action displayed on 1 changes to Solid Barrel. You press 1 again (whenever you feel like it, of course), you execute ONLY Solid Barrel. Now the action changes back to Keen Edge. The action would also change back to Keen Edge whenever your combo breaks (taking too long, starting a diff combo, etc.). It's exactly like pressing 1-2-3, except now it only occupies 1 button instead of 3.
The Solid Barrel Combo action already works like this! The action changes based on where you are in the combo, but it only occupies one button slot! That is all we want! If you're okay with how Solid Barrel Combo works, then I don't see why you'd be against the same concept being applied to other combos. Can you imagine if Solid Barrel Combo was three separate buttons we had to put on our hotbars? Yikes. And yet, I'm sure some people would prefer it as 3 separate buttons! And I fully support their right to be able to do that if they want to (though I guess you could simulate that right now by just putting Solid Barrel Combo on your hotbar 3 times)!