Thinking back on when I made this measurements specific, it shouldn’t matter if you have a unique tag except to differentiate from like measurements from a different device, and the $device tag should cover that. So yes, drop all the measurements made after the initial bulk upload, and then restart uploading using the same groupings, so that all of the measurements in the group end at the same time.
As far as the timestamp of a maximum measurement, it’s a time-series database, so every measurement has an associated time. I’m not going to look up the syntax, but you can query for the maximum value of a measurement and the returned Json should include the timestamp as well as the value.