You can't overwrite old data. You should save the time of the last event from each file and only send newer events. If you have a chat file per day, you don't need to store this data for thousands of files, just the ones that changed today.

There isn't a way to get the timestamp data to be shown in the cached view beside each message.

If you don't want to handle a file, return E_FAIL instead of S_OK, that will tell the crawler to send the file to GDS for indexing.