You don't go back by the damage interval, necessarily; you go back by six seconds, because that's the smallest amount of time we can deal with under the resolution of the system.
Conversely the smallest amount of exhaustion we can deal with under the resolution of the system is 1 level. Either way is equally valid.
For that to be comparable to the starvation example, you could figure out how much damage the fire would do to you in a day, and Death Ward would have to un-do all of that damage.
Well, no. The fire mechanic has a time-frame that it operates on: it deals damage at a specific time each round. That's what I'm basing it off of.
Similarly starvation deals damage at a specific time each day.
To more accurately line up with what you're saying, I would have to work out how long that 1d6 fire would take to inflict damage, and have the PC die then, most likely before they even start their next turn.
Only using six levels for Exhaustion, and only applying it once per day, is a book-keeping measure for easy adjudication at the table. The in-game reality which it's attempting to model does not actually have six discrete steps that only increment once per day.
6 second rounds are a book keeping measure for easy adjudication at the table. Lots of things are for easy adjudication at the table. The entire "ridiculously short time to starve to death" thing is just so that a DM doesn't have to do math.
To me, the simplest adjudication at the table for this specific event is to apply each spell and effect as the book says: did you suffer an effect that killed you? Yes? Then the thing that killed you didn't happen. Fast forward to the end of the next day and starvation kicks in again.
Mind you, I'm not sure it matters in the slightest. I've never seen a character come close to starvation, even when playing darksun.