I think you can setup your first scenarios but i am not sure. why you want srdf pair before data come in picture?  for the second one you have to spil the pair, because OR need independent device as a control. makesure you mark as a not ready after split.
1) We are planning to migrate 3 devices from site A to Site B using Open replicator. Before moving the site A devices to site B we want to set up a SRDF/A relation for the 3 devices  between Site B and site C then start the OR sesion between Site A and Site B. Is this setup possible.  2) Also, we are doing the OR cold push from site A to Site B using BCV's on the Control side, so i think to do the OR session between site A and Site B do we need to split the BCV devices and then start the OR session. Please let me know regarding these 2 scenarios. 
Open Replicator and SRDF