"Can we say 'language representation that is not a transcription of a spoken one'?" Of course: sign languages. But sign languages have even more "phonemes" than spoken languages do, so this doesn't help the hypothesis.

And yes, "A sequence of symbols could have meaning without mapping to sound": that's what hieroglyphic writing systems were, more or less. Again, more distinct symbols, not fewer.

"Do hobo signs count as a language?" Depends on how you define "language". As most linguists would define language, though, the answer is no. All normal spoken or signed languages have oodles of structure (grammar). Pidgin languages probably do not, but that's just the first generation or so, after which they gain structure and are technically creoles. (Some creoles have the word "pidgin" in their names, like Tok Pisin of New Guinea, and Hawaiian Pidgin, but they're complex enough to justify the term "creole.")