Dear friends,
Perhaps I'm missing something, but prof. Slaje's argument would work only with the assumption that the verse/text was mediated through Śāradā only and no other script until the printed form. I am completely unfamiliar with the manuscript history of this text. Moreover, I am looking at an 11th c. Śāradā ms. right now (do we have anything slightly earlier?): while ccha and stha are clearly different, they are not _that_ different and I can see how in a hurried hand they could've been confused.
As for sgrib byed/med, a Tibetan could have thought that such an inauspicious word is not apposite for a maṅgala/pratijñā verse, but perhaps it is nothing more than a psychological slip: while sgrib byed and sgrib med occur more or less at the same rate (quick grep through the Bstan 'gyur yields 68 vs 67), a search for sgrib pa med pa vs sgrib pa byed pa yields 919 to 1.
Best wishes,
Peter