I have several northwesterns and have kept the species over many years since 1978. I don't recall ever seeing wrinkles when they bend. I only see that in snakes that don't normally have wrinkles, when they are dehydrated and/or malnourished.

I have also seen infections cause one cloudy eye but it doesn't usually just clear up on it's own.

I have never got a NW to eat slugs. Well, there was one time and it was a disaster. Poor thing couldn't close it's mouth properly for a week.

Mine always readily take night crawlers or earthworms. They will also take pacific chorus frogs or parts/larvae and sometimes fish. I have kept them successfully for years on nothing but night crawlers with a dusting of calcium/vitamin supplement every few feedings. They have to be fed often though since those foods are mostly water.

An abnormal daily cycle is not recommended. Garters are very sensitive to light cycles, not just temperature. A regular day/night cycle timing and length appropriate for the snakes latitude and uninterrupted darkness and cooling at night are very important.

If the snake needed to shed some time ago and for some reason didn't, or went badly, the old skin can become fused to the new and cause wrinkles. Once it's been that way for a while, placing him in a shed box for a few hours isn't going to do anything.

I also know nothing about any housing, lighting, heat, etc. that the snake receives. Please describe.

Northwesterns are almost as common and numerous as the worms they eat and it's easy to find plenty of healthy ones. I wouldn't bother keeping it if it's having problems.