If you were feeding 4-5% and only able to shoot once a day, feeding 7-8% might let you shoot consistently twice a day. It might help moderate the sudden drops, too. Since many cats go lower at night, feeding a very slightly higher carb dinner might help prevent him from going too low when you're trying to sleep.
Lantus, according to the manufacturer should not be diluted, so it is difficult to measure precise doses under 0.5 units. ProZinc is already at a lower concentration (U-40, or 40 units per mL), so micro-dosing can be done as low as 0.2 units when using a conversion table with U-100 syringes.
A chronic URI may impair the ability to smell, which may make food less appetising. This could reduce appetite and thus cause weight loss. If it is feline herpes, adding the amino acid lysine may help supress it. If it is some other respiratory infection, IDEXX offers several respiratory panels on bloodwork. That was how I discovered Spitzer had a form of bartonella and got him on the proper treatment. He was always mildly stuffy, not severly ill.
If there is an ongoing source of infection, that could be what is elevating the glucose. Treatin that could eliminate the need for insulin, given how low Bear is already.