It won't be both of them though. Ideally, it won't be either of them. Ross is a middle reliever and Walker is the long man. If someone gets hurt one of them will step up just like Strahm did in 2023 and/or step back in 2024 like Turnbull had to. The latter situation obviously didn't go well but Turnbull was probably an injury waiting to happen regardless. He certainly had good reason to be unhappy but he never was publicly (even despite his previous history with Detroit). Ross explicitly agreed - and was paid - to take this role, so it should not affect him or the dynamic. And Walker's been a good soldier.
I think the odds are still that Walker isn't in the picture. But if he is that means the Phillies value the emergency depth more than they do the 8th bullpen spot or any chemistry concerns. If he is lights out and they actually want to go with a six-man, that's not necessarily a bad thing. It would still probably lead to him being traded, and you get the same number of innings from the spot (6 IPs once a week instead of 2-3 twice a week).
I think they are pretty firmly committed to not using Painter until after the All-Star Break. He's probably going to skip starts and have an innings limit in Allentown too. He won't really be ready and they don't want him to come up and just start ripping it on short notice. Of course, stuff happens.
Bullpen games work in MLB in 2024, and what's more, most times the Phillies didn't do one last year they turned into one anyway.
Don't really see why you'd piggy back the sixth starter. It would be Walker. You let Ross relieve, on a normal reliever's schedule. You can't have only six relievers