Ukemi sometimes translates as the art of falling away from harm.
I believe this translation iof the wordisquite mistaken Taking ukemi is another way to practice kokyu royku, "Abdominal Breath Power".but I think your idea of ukemi as a WAY TO DEVELOP HARA IS CORRECT
THE WOR "UKERU MEANS TO RECEIVE THE WORD UKEMI IMPLIES RECEIVING WITH THE BODY ESPECIALLY HARA
If the essence of executing an Aikido waza/technique is to be balanced, centered and in control of oneself, correspondingly, the essence of executing ukemi is also to be balanced, centered and in control of oneself to the greatest degree possible. Paradoxically, the way to maintain the greatest control of one's balance, is to give it up! Walking is a process of deliberately allowing yourself to fall, generating momentum in the direction you want, then catching your fall. Running is actually the process of throwing yourself forward and then spinning your feet fast enough to almost catch up. In neither walking nor running should one loose control. Ukemi is the process of giving up one's balance, but not one's control. Uke can still retain control, at least of their own body, enough to ensure a safe fall or take-down.