I think what they did was far, far more commonplace than you'd think nowadays. In many ways, Madeleine McCann's disappearance was a game-changer in how folk viewed leaving their kids.
The nanny service that was available was essentially the same as what the parents were doing - regular, routine checks on the kids rather than an adult sitting in with them.
I remember pre-Madeleine McCann having a weekend with the wife and my son, who at that time was a baby, at the Roxburghe hotel (which IIRC is in the Borders), I might have the hotel wrong but it's not that relevant anyway.
We were staying in an outer building adjacent to the hotel, or off the main hotel at least, and because Josh was such a nightmare for sleeping we couldn't get him settled (our plan was to take him with us in his pram, and eat while he slept in the dining room beside us).
As he was awake, we asked the receptionist if it was ok to order our meal as room service - we didn't want Josh crying while folk were eating their meals. The receptionist offered to check in on him every now and again to let us go in for the meal. We politely refused and took the meal to the room, but it was clearly not frowned upon to use a service like that.
In which case, you can see why a group of adults who were happy to take turns to effectively perform the nanny service wouldn't think it was a big deal, and while it's a big "how could you?!" nowadays, back then I think loads of folk would have done it - especially somewhere like Luz.
Re the argument that they weren't prosecuted because of their class or wealth, I can't remember seeing a ton of cases were poor folk have been prosecuted for it, either so I think it's a bit of a straw man argument for me.
I don't think they murdered her either, I think the implications and how many people would need to have been involved in covering it up makes that the most unlikely scenario. There's also the fact that they both had professions that were dedicated to saving lives, and I know you could say the same about Harold Shipman, but I think in general that's a good indicator that they care enough about human life to not become murderers.