We can grab instagram user IDs with the usernames, but -
can we, in opposite, use the IDs to grab the users' other data?
I think you might be a Supermetrics for Google Drive user instead of Data Grabber user? (This was posted to Data Grabber category)
In Supermetrics for Google Drive, you just need to change the query type to "Instagram user posts":
After that you can see which fields are available to pull from Instagram API.