I’m not sure if I could make it clear with my poor English, though, I’d like to try. First of all, the meaning of the sentence: 喜怒哀楽がはっきりしているぶんだけ、まだしも夜々のほうがわかりやすい。 (The narrator thinks that it is not easy to understand both of the two person, but…) The narrator can understand Yaya any easier (than whom we can’t see in the sentence), because she(Yaya) shows her emotions clearly at least(, while the other doesn’t). From the quoted sentence, we can see there should be another person compared with Yaya who is mentioned earlier. So I added brackets to the implied parts. About ぶんだけ: I think you’ve gotten its meaning. In this case, it is something like; “As far as the point that Yaya shows her emotions the more clearly,( the narrator understands her any easier than the other.)” In this sentence, this “ぶんだけ”-clause shows the reason of the main clause. About まだしも: I suppose your explanations is good for this kind of sentences: Xにくらべれば、まだしもYの方が*ましだ*。 Y *is any better/more preferable* as compared with X. (But neither of them is the best/most preferable.) This “is better/preferable” came from “ましだ” If it is: Xにくらべれば、まだしもYのほうが*早く走る*。 Y *runs any faster* as compared with X. (But neither of them is the fastest runner.) As this example shows, まだしもis not used only for the comparison of preference, but for various comparison. And the more important thing is that it always implies neither of the compared two is the superlative/worst, or to say the both are not sufficient enough. Adding to this, in your sentence the ‘X’ is not shown in the same sentence. But, the words “まだしも” and “夜々(Y)のほうが” imply there is another person who might have been mentioned earlier. So, まだしも夜々のほうがわかりやすい。 = Yaya is any easier to be understood as compared with the other. = (The narrator) can understand Yaya any easier than the other. I hope this made some sense.