There is hope that my pancreas will recover somewhat, since there is a theory out there that fat stored in the pancreas is part of the problem with how well beta cells function and intermittent fasting and low carb has decreased the amount of fat stored in my liver so it's possible it will work on the pancreas over time as well. That would really be a reversal of T2D if it happens.
Does a "keto" diet regenerate beta cells? Can someone with "reversed" T2D eat an isolated high carb meal and experience a normal glucose response? afaik, no and no.
So it does improve, but it's unlikely to completely go away. In my case it's likely from improved insulin resistance.
To answer your question, I wouldn't say it "crashes" back. It's still a concern, but if it's controlled, you can eat some cake and cookies and you'll stabilize in a few days. It's when it's a long term habit, that it's uncontrollable (at least on my case study of one).
With drugs you have very accurate protocol.
But when it's about changing your life style, you need to learn about what you atr doing, and regularly adapt. It's not a passive process and it's why few people do it : you basically take responsability for the ride.
From what I have seen/read, it seems to me that Type 2 is caused by a recurring assault on your metabolic processes through significant carbohydrate over-consumption. That damage accrues over time which is why it usually takes decades to become a full-blown Type 2 diabetic. It would therefore take years to fully "fix" it in a person - and that is assuming the accumulated damage is actually reversible. I believe there are some people who have damaged their bodies too much for it to actually be fixed. And on the other hand I rarely ever hear about anyone actually being full keto for years on end, providing the requisite time for their bodies to adapt and heal. It's not a quick process. The less damage, the quicker the improvement and the shorter the total time to "fix".
As to your original, direct question - I doubt it would come crashing back after just one week of bad habits. But that also has to do with the glycemic storage in your body. You muscles hold (I think it was) around 100-200 grams of potential glucose and your liver can hold something like 400-500 grams. If someone is low-carb for even a short amount of time, these "storage tanks" (as I refer to them) in the body are basically empty or close to it. As you consume garbage the excess that is not needed is added to these tanks. Once the tanks are full, the overflow goes into your blood, raising your blood sugar above normal levels. So you would have to consume somewhere between 500-700 grams in excess carbohydrates before you even started overflowing into your blood stream to have elevated levels. That's pretty hard to do in a single week. And the metabolic damage doesn't magically come back overnight. It's kind of like if you had a really bad sunburn and let it heal. The next time you went out into the sun you skin doesn't turn red immediately - the damage has to actually incur/accrue.