Yeah balance is perhaps not quite the right word, because you don't want a mechanic here that is so finely tuned that you're calculating the optimal path. More that there should be just enough hindrance so that instead of long resting after every encounter you are made to think a bit more about pushing the exploration with your current remaining spell slots etc... and I do think that is the aim here, to just make you re-think pressing that long rest button and that's all it needs to do right?
This, exactly. The food supplies don't have to be super-scarce, it only has to make people less willing to long rest after every fight.
I don't know how the "you get some spell slots back for a shallow rest" mechanic works, but I suspect that you won't be able to repeatedly shallow rest to be fully restored, it'll likely only restore you to a certain amount and if you are at that amount or higher, you don't gain anything.
