After the first file is ingested, there should be no delay if other files arrive in the interim.
Sorry but can you clarify. Are you saying that PM drops the delay after the first file in the live ingest session or that additional files don't delay the arrival of the first file and that all files are subject to the delay?
If it's the latter could PM set a trigger to live ingest a file based on the size of the first file (probably would want to check that the first frame isn't a black frame - flash didn't fire). So, say the first file is ~27MB, PM moves subsequent files when they have had enough time to deliver say 50MB or some other reasonable amount of data from the time of detection. Would that speed it up?
Copying an incomplete file would be disastrous
True. I suffer these disasters on a regular basis due to Capture One Pro and Canon utilities dropping the camera connection. I've found a much more reliable bit of tethering software, but would like to maintain at least some of the promptness of it's less reliable cousins.