Hi Melissa,
I am sorry you're struggling with getting Satan's BGs under control.
After taking a look at his spreadsheet it looks to me as if you are dosing based only on the higher values, giving more when they don't go down, rather than letting those severe lows guide you.
Nadirs are the guide when using Lantus or Levemir. A low of 28 means a serious dose reduction is needed. Going back to the same dose which caused the 28 within 5 days of it happening is going to repeat the cycle. Please read the Pet Diabetes Wiki page on Rebound.
http://petdiabetes.wikia.com/wiki/Rebound The graph there should look familiar. There is also this page
http://www.indulgedfurries.com/petdiabetes/somogyi.htm with another helpful graph.
The safest and best method with Lantus or Levemir use in cats is starting with a low dose and increasing the dose gradually
if the nadirs indicate an increase is necessary. If you lower his dose to 1U and hold it for 5 days, you should be surprised with the results.
One thing which might be helpful is to set aside what you know/have learned about human diabetes. While it may seem logical to increase the dose when blood glucose values are in the 300s and 400s, those values aren't as dangerous for cats as they are for humans. Although not desirable in either case, when we desperately think we need to get high BGs down by raising the dose, the result is often counter-productive because the cat's body reacts so extremely to even a small overdose. The rebound hormones involved is why you saw the 500 on Wednesday, 500s rarely occur without them when a cat is on insulin, especially when using either Lantus or Levemir.
Yes, that means he had a low overnight that night. The downward trend was evident at +5 because the 457 preshot was another "bounce" from rebound hormones.
Some may argue that you have to keep raising the dose in order to break the cycle of rebounds. In this case I think that would be dangerous. He is obviously responding well to Lantus and just needs some fine tuning in order to level out the response.
The diet changes you have made have a lot to do with why he's responding so well to the insulin. You have removed his carb load and now his body doesn't have to fight that battle and the insulin has an easier job to do.
And you do too!