Hello and welcome! Refrigerate the Lantus right away. Those instructions are not for cats! The pen needle mechanism (used by humans) doesn't work well if in the fridge, but since you are using U-100 syringes (I presume), that's not an issue for you. Lantus pens should be able to be used to the last drop if kept in the fridge. Humans at their doses use up Lantus before it would go bad if kept out of the fridge.
As for the AT, since we don't know how to convert AT to human meter numbers, we just use the same numbers at face value for SLGS. SLGS was developed back in the day, not so long ago, when AT's didn't exist and everyone used human meters.
As for dose, I think you absolutely can go to 2.5 units of Lantus. We take the existing dose of insulin in mind when switching from other insulins, and 2.5 of Prozinc wasn't getting Tibby anywhere dangerous. Some cats have a stronger reaction to a new insulin, and if you were seeing blues or greens on 2.5 units of Prozinc, I'd be more cautious.
