Glad you got the cystocentesis done. Hopefully that will help get the right antibiotic for warren.
There are a couple of points that I haven't seen anyone address yet.
I think it's helpful to understand how Lantus works. It's a depot insulin, so when you inject, some goes to work immediately and some builds up in the body and slow releases. The size of the depot is related to the size of the dose. When you increase a dose, it can take a few cycles for the depot to build up. When you decrease a dose, the larger depot can take as long as 6 cycles to reduce to match the new smaller dose. During that time, the old size depot can continue to "give" and lower blood sugar.
That's why changing a dose too often, like with a sliding scale, can work against you. To see what the current dose can do, you want the dose and depot to be settled in. Think of it as needing a calm lake to evaluate the dose, and changing the dose too often is like rocking the boat.
Here's a good description of how it works.
http://www.diabetesclinic.ca/en/diab/2treat/lantus_vs_levemir.htm
Lantus dosing is based upon how LOW a dose will cause the cat to go. It can be hard to change after you learned on vetsulin, which isnt a depot insulin, but lantus dosing isn't usually adjusted based upon the preshot test, unless it's too low to shoot. With lantus, it's really important to get some midcycle tests in so you know how low he's going. Sometimes high numbers are caused by bouncing from low numbers.
If you could add on 1 more test per day, it would tell a lot about what's going on with him. Doesn't have to be at a particular time, just sometime between the shots. If you imagine the spreadsheet as a jigsaw puzzle, getting a sprinkling of tests throughout the spreadsheet will help reveal more than a stripe down the middle. What you have with preshot tests are sort of like having the edge pieces of the puzzle without anything in the middle.
You commented about the meter and him getting to 40's...Lantus is gentle and he may very well have been in the 40's without hypo symptoms. That's not at all uncommon in the Lantus ISG. You don't want him thete, but i wouldn't assume your meter was wrong.
If he gets another antibiotic you might want to get a probiotic to prevent diarrhea. Most cats like Fortiflora. You just sprinkle it on their food. It's available only from vets or online.
Anyway, I've written you a book, but hopefully some of it will help you help Warren. Keep asking questions! There is a wealth of experience on here and people will keep helping you.
Hang in there!