I’ve been making a bit of progress on Methodius today. I learned from an academic that no published text of most of the Slavonic Methodius exists. The manuscripts are 15th century or later, as we already knew; and I suspect that the Russian volumes (from 1877 on) are simply updated versions of these. A couple of other interesting details when I get permission to post them.
I’ve also been looking at the 1930 edition and translation of On freewill by A. Vaillant in the Patrologia Orientalis 22. Vaillant tells us that the Slavonic is so literal that you can see the Greek behind it; and indeed even the marginal corrections in the exemplar! As a result, his edition is a little confusing: he prints the Slavonic; opposite it he prints a Greek text, his own reverse translation from the Slavonic; and underneath a French translation made from what he believes the Greek was. He checks the Greek against the surviving Greek fragments.
If there is no published Slavonic text, we might have to make do with translating some Russian versions into English. But I would hesitate to spend money on translating a translation — it doesn’t make sense. It would be better, surely, to obtain a manuscript reproduction and translate from that.