This all seems to be much ado about nothing.
Melito did not “reject” the deuteros. Nor did he “affirm” the proto canon. He had his very own personal canon of Scripture, which is not the same canon as what anyone today uses.
At the time that he lived, this was perfectly okay - most people at that time had very limited access to books, and there was no such thing as a “canon” of Scripture, yet.
The Church itself (taken as a whole) was using the entire 46-book canon of the Old Testament when ever and where ever that was available, but obviously (since books were difficult to get in those days) it is not likely that every parish or even every Diocese had a complete set of the Old Testament, or a complete set of the New Testament, or even anything close to a complete set - they were doing the best they could under the circumstances of the time.
