This does not answer your question, but it is relevant so I'm leaving a comment:

First, we have a 2021 paper which seems to indicate that Intranasal Insulin does affect systemic glucose control:

A Comprehensive Review of Intranasal Insulin and Its Effect on the Cognitive Function of Diabetics - PMC

Intranasal insulin has a rapid mode of action while effectively controlling postprandial hyperglycemia. It has also been proven to reduce hypoglycemia and insulin resistance problems, which seem to be the main adverse effects of using conventional insulin regularly. However, due to the large dosages needed and high incurring costs, Intranasal Insulin is currently limited to use as adjunctive therapy along with conventional insulin.

One unique feature of intranasal insulin is its ability to directly affect the central nervous system, bypassing the blood-brain barrier.

it has also proven to play a role in improving the cognitive function of diabetics, especially those who have Alzheimer's or mild cognitive impairment, as decreased levels of insulin in the brain has been shown to impact cognitive function negatively.

However, it does come with its limitations of poor absorption through the nasal mucosa due to mucociliary clearance and proteolytic enzymes, our body's natural defence mechanisms.

Seemingly in contrast with the above , we have an older (2013) paper that reaches the following conclusion:

Enhancement of vasoreactivity and cognition by intranasal insulin in type 2 diabetes - PubMed

Conclusions: Intranasal insulin administration appears safe, does not affect systemic glucose control, and may provide acute improvements of cognitive function in patients with type 2 diabetes.

Finally, we have this interesting 2022 article:

Trial finds insulin nasal spray may slow age-related cognitive decline

The results of a small Phase 2 clinical trial offers promising signs that daily doses of an insulin nasal spray could be used to slow age-related cognitive decline. The trial found the treatment to be effective in elderly subjects both with and without type 2 diabetes.

The idea is that nasal delivery of insulin sends the hormone directly to the brain. This means intranasal insulin (INI) wouldn’t be a replacement for insulin therapy in diabetics, but instead it works to modulate insulin signaling in the brain.

Prior human trials with intranasal insulin have delivered mixed results. A moderately sized trial testing the treatment for 12 months in patients with either Alzheimer’s or mild cognitive impairment found no benefits. However, this new research hypothesized the treatment could be more effective in preventing general age-related cognitive decline instead of acute dementia.

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