The fundamental problem, as I see it with the Didache, is given by the fact that we are still missing a major critical edition. All that we have got (SC, FC ...) are editiones minores.
These pretend to give a text which, in fact, is only mirroring fourth century recensions of a text. As all editors admit, the existing recensions that we have are so divergent that an editio maior would need to put side by side the various recensions so that one can recognise the differences and variations (much as we have it now with Aristides). So to the question of origin and date of the Didache one would first need to establish the relation between the various recensions to dating and locating of them, before one moves into hypotheseis about the potential earlier versions. The strong differences between the recensions underline, indeed, that this text was heavily revised, being a non-authored, partly catechetical, liturgical, practical, juridicial document which, as one can see from the reuse in the Apostolic Constitutions, has been seen as a building block for further use.
The question of the right text has an enormous bearing, for example, on the discussion I have with many New Testament scholars. When they point to the use of Matthew in the Didache, they refer amongst others to what scholarship calls the 'gospel insertion' (SC Did 1,3-6) which is almost entirely taken from Matthew 5 (Luke 6 par.) - yet this first 'Christian' part is present in H,CA,O,G, but is missing in KO,E and L, neither is there a parallel in Barn. 18-20, and therefore left out of the edition of the Did. by Klaus Wengst, for example. B. Layton has already pointed out that the Didache, as we have it (also with this insertion), is a harmonised version, where Matthew and Luke have contaminated the text of the Didache. The main argument against this text passage as part of the earlier version of the Didache is that there cannot be given a reason why the KO would have left out from the Didache this passage which sounds most Christian.
Now, on such difficult grounds, a careful dating (locating not yet possible in my eyes) is given by Wengst: Terminus post quem is Matthew, terminus ante quem is Clement of Alexandria. As Wengst assumes an early dating of Matthew, he opts for the beginning of the 2nd century - but here comes the circular argument, as the Didache is being taken by the same scholars as the hook for the early dating of Matthew. As a result, we only know of a relative dating (which is even more dubious, as we do not know to what extent our versions that all date from the fourth century and later, are harmonised versions): Didache after Matthew and before Clement. In addition, as the Didache assumes already a certain separation between Jews and Christians and seems to be prior to Justin (all with huge caveats reg. text basis), it seems that anything that claims a date earlier than the mid second century is loaded with the burden of a heavy hypothesis. The Didache can certainly not provide the basis for a first century dating of Matthew and the other Gospels.
