It is unlikely that either of these assumptions will prove to be false in the general sense. It is possible, though, to find examples of texts which violate one or both of the two assumptions. It is easy to imagine an author intentionally misleading the reader; for example, mystery authors often do this to allow the reader to experience the same deductive process which the characters of the story are going through. But this is a local example--in the end, the author is still expected to provide the reader with enough information to comprehend the story. On the other hand, there might be authors who intend their works to be incomprehensible for one reason or another. I assume these are enough of a rarity to not cause serious problems for the theory I am presenting.
It is much easier to imagine a text which contains only known concepts. In some cases, this will result in a straightforward comprehension. However, this fact would not cause problems for the theory being presented here; it would simply be a degenerate case. It is still likely, though, that even texts which contain no unknown concepts will be read in unique ways. Readers bring their own background to the comprehension process; so, even if a reader finishes a text and immediately decides to re-read it (thereby making it highly likely that no new concepts are present in the second reading), the comprehension which is produced is likely to be different. For instance, the reader will generate different expectations during the second reading than during the first (e.g., see [#!read:ram1!#]).