That's a good rule of thumb whenever you travel with a large group - there's no law that states you have to do everything together, and that includes eating every meal. Now, if the entire party is on the same reservation, the two who've had surgery will still have to be on the same dining plan (if the rest of the party is on a dining plan). But they'd still be able to use their credits at another restaurant where they could order ala carte. Ideally, their reservation is separate from the rest of the group so they're not beholden to join in on a dining plan, because it sounds like they'd spend far less ordering smaller meals or sharing entrees, stuff like that.
I understand why the buffet restaurants don't offer any ala carte options, human greed being what it is and all you know people would order "just a salad" and then "pick" off the plates of other people in their party paying for the buffet, but it really would be great if they could work out a way. I love a buffet myself but I also recognize that I'm a bit of a lardo in a nation of lardos, so people being able to eat together with the option of being a glutton or being selective in the same restaurant is a great option to have. I know there are some options, usually an ala carte menu that has a separate buffet (breakfast at The Wave) or some kind of all-you-can-eat family style option (the skillet at Whispering Canyon Cafe).
I wonder if people would be willing to dine at a buffet and eat from an ala carte menu if they had to somehow be "singled out" with something like a literal red flag on the table to denote ala carte patrons, so the table might be more closely monitored to make sure no one is a sneaky Pete. I wouldn't opt to do it, you have little enough privacy walking around WDW, but I'd bet lots of people would if it meant kids get to see characters they'd like to see and families could still all eat together.
I'm no expert but I've been exactly where they are.