descent with modification

[pfeaster]

The first important point is that far more potential variations than just the changes between [Ed], [Eo], and [Ey] exist. For example, [chedy] could change to [chey], but also to [shey], [cheedy], [keedy], [tedy], and so on. The relation between words containing [Ed], [Eo], and [Ey] is therefore more complex.

That's true if what we're interested in is the full range of words that could result from modifying each source word according to your model.  But in each case, a source word containing [Ed], [Eo], or [Ey] would still have that specific sequence either kept or changed when copied, regardless of whether other parts of the same word might change or not.  Unless your hypothesis posits that a change to one part of a word makes another simultaneous change to another part of the same word less likely, I wouldn't expect this to make a difference.

I compared those three sequences because they happen to represent the three highest-probability transitions from [E].  However, if you prefer not to compare those three features with each other, there's still the grayscale display limited to [Ed] itself, which I believe shows more or less the same thing.

What I'm trying to assess is the hypothesis that [Ed] becomes more common over time as the cumulative effect of a tendency to switch towards it more often than away from it (please correct me if that misrepresents what you're arguing).  Could that happen between pages if it didn't also occur to some degree within pages?  It must happen somewhere if it happens at all.  I suppose it's possible that the writer favored [Ed] when copying words from other pages, but not when copying words from earlier on the current page -- but that would introduce an added complication.

An alternative hypothesis would be that each page starts out with a slightly different set of transitional probabilities (either as a consequence of how it begins or from some other cause) and that these manifest themselves through the mix of word forms found on that page, with certain "types" of words being more or less common because of the different transitional probabilities implicated in forming them (rather than the other way around -- i.e., the different probabilities result from a page-specific mix of word "types" that arises in some other way).  How would you go about ruling that out?

Secondly, not for all words containing [Eo] similar words containing [Ed] or [Ey] exist. There are for instance also words like [cheol], [cheor], [sheol], and [sheor] which are not directly related to vords like [chey], [chedy] or [cheody].

In this context, variation among [cheol], [cheor], [Sheol], and [Sheor] would count as "no change": [Eo] remains [Eo].  If [Ed] is steadily increasing, it should still increase relative to any unchanged proportion of [Eo], shouldn't it?

Third, each page essentially starts fresh. Take folio 103r, for instance. <...> this doesn’t imply that the same vords are frequently used on f103v.

I've been assuming for this discussion only that the words at the start of each new page have been copied from some existing page(s).  If new pages didn't build on previously written text, there couldn't be any cumulative effects.  I believe your arguments at least require Currier A text to have been available as a source for all Currier B text, and for earlier Currier B (with a lower proportion of [Ed]) to have been available in turn as a source for later Currier B (with a higher proportion of [Ed]) -- correct?