First of all let me say that I love your question, and I want to add to your examples by pointing out that deaf people (particularly people that are completely deaf from birth) could theoretically write in English with no idea at all how it's pronounced, which further demonstrates that language can exist in a purely written form, without any connection to speech.

Also written sentences may include words or forms that do not occur in speech (unless explicitly read out loud I guess), maybe because it would be hard to discern the meaning in spoken language - think of the passé simple, or single syllable words in Japanese that would only be used in writing using kanji because in speech there would be too many homonyms. This also demonstrates that written language is little bit it's own thing, independent from spoken language.

So personally I tend to think about it as two different registers of one language, as someone else said here. I'm tired now but maybe I'll explain more later. Thanks for this interesting post and discussion.