As a former engine calibration engineer for an OEM, I'm enjoying your posts on working on the cal. I'm not familiar with what the various aftermarket systems offer for tuning knobs (i.e. their software), but what you describe for the IAC control being ok for a bit and then going out of control sounds like either the gain on it is too high or the rpm signal feeding the IAC/idle logic needs a heavier filter on it to prevent the IAC from chasing it so much. Classic control problem! Does the software have a "proportional" idle spark control where the spark adds/subtracts based on how far the idle rpm is from the commanded rpm? If so, since that acts much faster than the IAC can (IAC has to feed/take away air to the intake, air has to get ingested, then combusted, etc), the gain on that can be more aggressive and more effective to maintain a smoother idle.
Again, I don't know what knobs these aftermarket systems offer but calibration is/was a fun job!! Keep at it as you'll get there! And keep posting - enjoy following along!