Meh. I've been trying for years to figure it out. I was here today, once again looking for more info when I saw your OP.
Not sure what the problem is, other than I seem to put the game away for too long (awaiting bug fix patches) and DLC changes what I thought I knew. Upgraded to 4.21 last night and I'm starting over, again, because it seems a couple DLC (Crusade and Retribution) are required to fix things that the 4.21 upgrade patch didn't. Was looking forward to the AI Trade Offer update, but they are still just as idiotic as they were in previous versions.
But, with GC IV in Beta, maybe GC III will remain consistent for a little longer now. I'm not convinced 4.21 is a worthwhile update yet, but I'll give it a couple more games.