Ďaiki' could be understood as the use of the opponent's energy against him/herself (that is to say, using their energy toward our own designs which we can say are at the moment not in total agreement with that of the attacker)."
I would probably describe it differently. However, based on the above definition and in response to your question on "what is the relationship between striking and the tactic of "aiki?" A real simple answer would be to use the energy of the attack towards our own design I could simply stick out my tegatana at the moment of an attack thereby using the energy of the attack to creat a strike so to speak. For example, on a munetsuki strike I could energy off like with my tegatana extended causing the uke to strike himself against my tegatana (using his energy). If I'm accurate enough uke will strike his own pressure point against my tegatana thereby creating an atemi and likely pain compliance resulting in a dropping of the shoulder.
I'm using your definitions here as I understand them.
To me, however, the use of aiki would involve blending with the energy of the attack and redirecting it in a relaxed and purposeful fashion. A strike or atemi here would have to involve timing in such a fashion to not disrupt the energy which is necessary for the smooth blend while still causing the pain compliance. This requires timing and appropriate direction to keep from stopping the energy. Hard to explain but not too bad to do.