No. No one does. That's the problem. Google on "textual criticism" if you're interested. If you're TRULY interested, click here.
HMmmm...
I guess there ARE 'originals' somewhere!
Every "scientific" work has assumptions in it, whether you recognize it or not. Its an assumption that a scribe would make a change to a manuscript to make it easier to read. Its an assumption that the more difficult reading is to be preferred. Its an assumption that earlier textual support provides the weight of textual evidence for a variant. Its an assumption that geographical support should be weighted in textual evidence decisions. There may be good rational arguments why those assumptions should be accepted; but in the end, they are assumptions through which the entire work, including the conclusions and implications, is dependent on.
I've read and studied his work with Metzner, its a solid scholarly work ... but it's his view given his assumptions. If you hold his view, then fine; your are already accepting the implications to your faith and spiritual life.
But you can read a work like this, understand the assumptions, approve of the content within those assumptions; but reject the implications of the arguments based on theological method. Are there places in the text where 2 readings could be probable, where each can make sense in the context of the passage? Yes. Are there places in the text where those 2 readings would have a drastic effect on a major Christian doctrine? No.