Understand what you mean when you say ‘metabolism that horrible’.
For someone to not lose bodyfat while eating 500kcals think about what that entails.
Their body is so efficient, that they can sustain brain function, immune function, organ function, repair, growth, cellular processes AND all their daily movement and activity, on only 500 calories.
Phrase the question differently. Does anybody think your 200lb wife can have a caloric maintenenace as low as you outline? No.
Maintenance calories can get very low, look at late stage female bodybuilders, when they are down to ridiculously low bodyfat they might be averaging just over 1,000 calories a day, for a short period of time, but they ABSOLUTELY know thats happening. Their average body temperature is way down, their movement is sluggish, their brain is sluggish, their hormone levels are tanked and their bloodwork is absolutely NOT normal (all things that are part of that metabolic adaptation you claim you don’t believe in).
There may be an element of overestimating how much is being burned, but that would be a difference of a few hundred calories.
For reference Im 200lb and my maintenance is around 2,250 - 2,400. Yes I can say i workout intensely 4 times per week, yes i run around after my kid, but when it comes to actual hours in the day most of them are sat on my ass. On my ass n the car on the way to work, on my ass in the office all day, and on my ass eating after I get back from gym in the evening.
So perhaps lower the estimate of activity level and see what the new estimate TDEE is.
The rest of the difference is calories in. Full stop.