If I was retrograding a melody that ends on a whole note, should I count it as a first beat quarter during reversing it or should I count it as 3 quarter rests + a quarter? I've attached an example score to show you why I am asking this question: The retrograde A maintains the timing between the notes of the original melody and retrograde B is precisely an exact reverse of the melody. I am confused because I am not sure whether the timing between the notes should remain consistent during retrograde or not :/
If you are working to someone's set of rules, follow them! Otherwise, I think you can allow yourself some latitude. Do whatever works.
You can use imitation, write a strict canon, or anything inbetween. Likewise with the various transformations. They can be your servant or your master. Modifing the length of a first/last note is a VERY small transgression!
The retrogrades and inversion do not have to follow the same rhythmic values. The shape comes from the interval distances, not the rhythms. When I am composing and making my personal "dictionary" of my melody and the inversion and retrogrades, I don't use note values at all. I just write dots. Then I use whatever I want from there (whole or segments) to write my piece. The inversion and retrogrades are just tools to help you find source material to write your piece, unless you are trying to follow a very specific rule (whether someone else's or your own restrictions for the piece.)
I think a simpler way to look at this is to retrograde the entire 4 bars, beginning with the 3 rests. It makes it a lot easier to conceptualise, and generally this will work for definitions of "retrograde". If you want to start on a note rather than a rest, you can shift everything over by 3 beats later.