There is not enough information to say for sure why the pointing errors accumulated but this is not typical behavior seen with other ASCOM client applications.
1) How do I see what sync updates NINA (Automation Software in general) did to APCC? Specifically, what is theIf you need to correct the mount's position, I suggest you use APPM's "Plate Solve and RECAL" button on APPM's Run tab.
3) Are the NINA correction it does per point? i.e. Does APCC build an internal pointing model for every correction,APCC's pointing models are static once created. Sync's into the model just adjust the an offset into the model.
4) How does NINA correction integrate with the APPM pointing model?There is no integration, nor does there need to be. NINA is not even aware that there is a pointing model.
5) Is the effect of NINA corrections cumulative? It appears to be, I can't imagine APCC or NINA having missing aThere's something else going on if errors have accumulated to that extent. I suggest you turn off syncing in NINA until the source of the issue can be determined.