Are my adductors shorter when my knee is bent or longer when my knee is bent
You are best asking a Physio, doctor or other medical / biomechanically qualified person this question.
Instinctively, for what it’s worth, I suspect that
(a) it depends - if you are standing and the leg is hanging in neutral, your hamstrings will do most of the work / contraction; if you were lying or sitting down, your thigh, quads and associated muscles may be more heavily involved.
(b) other things being equal, it is possible, theoretically, to isolate muscles; so, there may be scenarios in which the adductor could be kept in a neutral position. but in truth the majority of functional movement involves the coordination of many muscles and muscle groups.
Just a couple of thoughts. Hope it helps.