"Upon further review"...you don't need anything fancy. You just need a loop-current detector relay per line (with a break contact) or, if the loop detector doesn't have a break contact, an additional relay per line with a break contact.
Put a loop-detector in the line BEFORE the Partner, and use it to operate a relay that opens the CA lead in the Shoebox that feeds the ringer(s) for that line. It will "not exactly" be an A-lead, it will be just an "RCO" (ringer cut-off) lead, which stays open as long as the line is in use.
Case 1: incoming call answered by shoebox --
Call comes in, interrupter starts, call answered by 1A2 set. A-lead knocks down ringing and flashing lamp.
Case 2: incoming call answered by Partner --
Call comes in, interrupter starts, call answered by Partner set, lamp continues to flash for a few seconds, ringer is silenced immediately.
Sometimes I amaze myself.
