Could those adjustments that you try to do to smooth out movement cause a lot of other issues?
Could that "buffering player movement" be why the game often stutters or has hiccups?
For example, when you talked about this
Our working theory is that remote interp delay / buffering behavior may be introducing unnecessary delay for some players in some matches.
Some buffering is required to smooth out player movement accounting for ping variance and packet loss, but the system is designed to minimize buffering as much as possible.
Players are using different routes to connect to the same server or game pod (AWS vs Riot Direct).
Server smooths out movements of enemies whenever they are visible, and the server tries to make things "fair" for players with different pings.
Could this be the cause of all the input latency whenever enemies appear on screen?
My biggest issue with "inconsistency"...