Back again with some general info and comments.
Ideally, I'd like to keep the new diet (seeing as he's currently fine), go off insulin, and monitor his blood levels to get good starting readings, but I know there can be serious problems going off insulin.
If a diabetic cat that needs insulin doesn't receive any at all, there is a risk that the cat's body may start breaking down fats for energy. This results in ketone production and puts the cat at risk of developing diabetic ketoacidosis, a life-threatening complication of diabetes. By way of general information, routine ketone monitoring is a wise safety precaution, particularly if a cat is running in high BG numbers (greater risk of metabolic dysfunction). Ketones may be monitored via urine (urinalysis strips) or blood testing (ketone meter, similar to glucometer). I see other members have already given you some pointers. Here are some useful resources for you to bookmark:
Tips for Collecting Urine Samples
Diabetic Ketoacidosis Information
Well, he hated the injections (He took the first one fine, then realised it was coming and freaked out every time) and is very stressed, so when he was a bit wonky I just assumed he was dealing with a lot of anxiety and stress. I asked the vet what signs to look for for hypoglycemia and he told me, but reassured me it was very unlikely. Bruce kept getting more and more "upset" as injections went on, and then today I was trying to pet him and he was out of it. I asked the vet if I could skip tonight's dose and he said "ok, but we need to be careful not to do that too often". The difference is night and day, he's so much more alert, he's purring, being a cheeky bugger, back to his normal self. I feel terrible that I might have been overdosing his insulin or managing his diet wrong or something. I'm second guessing myself that it could be stress, but he was just weak and glaring off into the distance while trying to get a scritch out of me earlier today, so surely not (usually he would've been crushing my hand demanding harder skritching, lol). What a mess.
With regard to some of the clinical signs you've observed in Bruce, it is valid for you to be concerned, to wit:
* OK on first injection, intent on avoiding subsequent injections, stress increasing with each successive injection. [Depot building, more insulin in system with each successive injection.]
* After administration of 7th dose, Bruce "out of it", "weak and glaring off into the distance". [Depot getting closer to 'full', greater risk of hypo.]
* Behaviour returns to normal when 8th dose skipped. [Possibly due to body no longer needing to 'fight' the insulin dose - speculating here.]
By way of general information, sometimes a cat may become lethargic or 'seem a bit off':
* for part of the time over the first few days of treatment with insulin as the body may take a little time to acclimatise. (Should be self-limiting, usually resolves after a few days.)
* when blood glucose levels drop too low, possibly accompanied by other symptoms like 'spaciness'. (BG testing will enable you to check whether BG is in hypo territory.)
* when blood glucose levels go through a steep drop into still-safe lower numbers from a much higher preshot BG. (With a Lantus kitty, likely cause would too high a dose. Again, home testing will help with this.)
* if the current insulin just plain doesn't agree with the particular cat. (Cat may 'seem more itself' when insulin dose is wearing off; usually resolved by switching to a different insulin).
* if it is throwing ketones or going into DKA. (Regular monitoring for ketones is best preventative measure. Extra caution needed if general BG level is high, or appetite is poor.)
If the insulin dose is set wrong or if the particular insulin doesn't agree with the cat, the cat may try hard to avoid subsequent injections. Bruce's behaviour with respect to receiving injections is definitely something to keep an eye on.
Conversely, some cats may initially be resistant to BG testing. However, when the insulin dose is set to a safe and effective level and they start feeling better, they recognise that the testing is somehow connected with that and they become much more compliant and accepting of the procedure. Some even start sitting in their testing spots of their own volition, waiting for their caregiver to check their BG levels and give them a treat!
Maybe also switch to a new vet? Honestly I'm so new to this and my vet is downplaying all the info I'm finding online and confusing me, so I'm just so lost.
Impression of current vet's performance:
* Based on the research-based dosing protocols we use here, the starting dose was set too high.
The above is poor but...
* Was aware that there was no initial capacity to monitor BG levels at home.
* Gave instructions to switch to food with over 50% less carbs.
* Does not appear to have given any weight to the effect of the simultaneous filling of the Lantus depot.
The above three constitute the diabetic equivalent of a perfect storm.
These are other concerns:
* Dismissive of client's observations of concerning clinical signs, discouraging attitude towards client's queries and efforts to learn more about feline diabetes.
* Disturbingly blasé attitude to assessment of risk of hypoglycaemia, and to treatment thereof.
Honestly, if this was happening to my cat and my vet wouldn't give proper consideration to very concerning observations about my cat's response to insulin, I would be actively looking for a different vet who had a better handle on feline diabetes fundamentals, and who was willing to work with me as a partner in my cat's care.
Mogs
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. I'm like so what was all the fuss about then ? Felines 
