The use of the preposition "in" in "in equal degree" in this sentence (what a mouthful) reads unnatural to me. Why is in used here instead of to equal degree? Although I am not sure if "equal degree" is idiomatic at all, I thought it should always be "to some degree".
The inflation of the printed word has been caused, no doubt, by the exponential increase in the number of those writing, but in equal degree by editorial policies. (source)
I'm only partially answering your question for now, since it's really difficult for me right now to nail down "why" it is "in equal degree", but I'm a native speaker of English (lived in the UK as a child for a good couple of years, and one of parents/ half my family are UK citizens), and the usage of the phrase is absolutely idiomatic (I would probably use the exact same phrase if I was writing, and intended the same meaning).
Some of the alternatives you give are possible as well, but the meaning is different - "to some degree" would work, but would mean ~ " also caused by editorial policies, with editorial policies contribution to the change being unspecified" , rather than "also caused by editorial policies, with editorial policies contribution to the change being equivalent/about the same as that of the increase of those writing" (which is the meaning in the quote you give).
Hope that helps a little already, I'll add to this answer if I can crack the "in equal" rather than "to equal" nut in my head :P.
