LT, Gumby:
I think we need define the words we use cuz we're getting a little mixed-up.
In the usual pattern a line is drawn between A. redaction period, involving writers, editors, and even glossators, as subject to literary criticism (i.e., criticism based upon internal, or content, evidence from the text itself) and B. transmission period, involving copyists or scribes, annotations or commentaries, as subject to textual criticism (i.e., criticism based upon external, or material, evidence from manuscripts and other witnesses). This limit corresponds to the fixation of the text, which is admittedly not an instant process (somewhat like the freezing of a river). So the exact line may be endlessly discussed: it's very difficult to distinguish a late (willful) gloss from the later editor from a (still later) scribal marginal note which eventually (and mistakenly) crept into the text.
The usual Bible editions try to stick to this (partly imaginary) line. The standard, btw, is different for the N.T. (where critical editions such as Tischendorf's, Westcott & Hort, or Nestle-Aland, make up a heterogeneous text from all extant manuscripts, which doesn't exactly correspond to any of them) and the O.T. (where the 10th-century-CE masoretic text, as standardized by the Jewish [pharisaic-rabbinical] community, is the main basis except for a few corrections). One could imagine a number of "mini-Bibles", ranging from the non-critical translation of one manuscript as it is (this has been done, in French, for parts of the Western Codex Bezae), to the hypothetical results of textual + literary criticisms: for instance, a translation of a reconstructed Q or Proto-Mark, which are offered in a number of scholarly works...
The only conclusion, IMO, is that there is no such thing as one original Bible text. Every stratum of scripture has its interest, and may be studied for itself.
PP:
IMO Tatian's Diatessaron (which includes some very late Gospel texts, such as GJohn's Prologue) implies the general fixation of the four Gospels, inasmuch as it didn't change them but was composed (and eventually rejected) as still another book. As for the logia sources such as Q or GThomas (and perhaps Justin's collections), one must take into account that they are not just compilations of disconnected sayings, but literary compositions involving a part of inventio (Kloppenborg) as well as the canonical Gospels.