To be honest, you won't find the answer you are looking for by changing the variables all the time. Changing shot times equals changing the dose - and lev likes consistency. You can't shoot it "as needed". You can't force a response that the given dose isn't going to give.
And he won't have organ damage in a few hours. It's just not going to happen. High numbers are not like dangerously low numbers that can cause immediate brain damage or death (if too low - and I am talking seizure inducing lows).
Probably having him checked for dental issues and having a CBC run is a good idea. Is he old enough for hyperthyroid to be a consideration?
If he is healthy and is eating a low carb diet, I think the only way he could swing from 60s to almost 500s is with rebound. I have looked over his SS (again) and I still think he is rebounding. I do not see a really good trial of either .5u or .75u in the past 18 months. By that I mean stick to the dose and a 12/12 schedule for at least 10 days. He had much better numbers on .5u and even .75u early on. Granted, that was almost 3 years ago (Sep-Dec 2009). When you started creeping the dose up his numbers began to rise so you dropped him back to .5u and he had beautiful numbers for a few weeks, but didn't hold quite as well as .75u did. Then you just rapidly increased to 1.25u and 1.5u and started getting 300s and 400s and has continued to get 300s and 400s while he has been over .75u.
He will, almost guaranteed, have worse numbers for 1-3 days once you try .5u, but that is what I would do right now if he were my cat. And if that didn't give great numbers I would increase to .6u and then .7u and let each dose settle for 7-10 days. The first increase should be to no more than .67u (half way between .5 and .75). I would test of ketones while I did this, of course.