Can neglecting type 1 diabetes for 1-2 years result in the development mental illness?

MI No; Dementia, Yes: Since diabetes mellitus (↑ed glucose) diffusely degrades a huge number of complex brain/body functions, including greatly accelerating atherosclerosis, it degrades brain function in many subtle ways, over time in far more blatant ways (e.g. Stroke), thus increases dementia. Mental illness: thoughts, ideas, behaviors which are destructive to self and others, a different issue, remains more complex.