1) BC_ARCHIVE archives entries from tables ADMI_FILE and ADMI_RUN, which are basis for the display of sessions in the delete program of SARA, depending on its status.
If that is archived you wont see the entry anymore in the deletion run (but the file itself is still there in your file system)
2) yes the entire archive session has to go in one shot, so archiving seems not to be an option for. then first try to set this one file as invalid and check if it still appears in the deletion program.