Within an inch of its life is a colloquial expression bordering on slang ............meant to be amusing because cleaning something is not life threatening.
I use words in an unusual form to make a reader take note as you did! One can be a little informal as it is Tevor telling the story and he is a teenager.
Yes goo goo means silly, amazed....like a kid might be.
Derision in this sentence means a good natured teasing whereas contempt would have been serious..... a fine line.
Excellent questions............I trust that my answers help............please feel free to question my answers. English is a language of many possibilities and one can develop one's own style by using words in a different way. No guarantee that I am 100 percent right!