The rightmost /De-cy, but with shorter teeth (move the topmost quarter-circles one step down and get rid of squares) looks to me like the real /De-cy the most.
However, I've tried the same treatment with /che-cy and /tse-cy, and their teeth (tails?) look limp. They look much better when row 0 is square and row -1 is flipped so that the sharp angle points inwards.
But this approach applied to/De-cy makes it look absolutely bloodthirsty. I guess squares in row 0 and sectors turned outwards in row 1 are the only compromise that can work for both.
The misaligned breve looks fine, to my surprise. I think even a single square over the middle column will look fine.
Could you try setting some running Cyrillic text in the latest edition of Tesserae? This would help a lot to catch any shapes that don't work that well.
@John Savard you're right, if ustav and poluustav are taken into account (I was thinking only about the civil type), then the rounded /р (or the eye-of-the-needle /р) is older. I checked "Apostol", the first printed book, and even the serifed vyaz titles there have a rounded /р.
I think we can all agree that the serifed /р is at least as old as the rounded /р, so rounding it won't make your font look more authentic. I would compare it to squeezing the umlauts into the bounding box of /A /O /U in a font you want to make more German. Yes, it's something German typographers did, yes, it's something that will make that German influence obvious in your font, no, it will not make a German designer choose your font over another unless they are actually in need of a font that does exactly that.