I realize this doesn't necessarily help you now, but it might be worth considering. We use GAT labs tools (GAT+, GAT Shield, Teacher Assist and GAT Flow). GAT+ is a webUI based tool that has a lot of the same capabilities as GAM (sometimes I think they used GAM as a template and simply built a UI around it and then continued to develop it). With GAT+ you can simply search every My Drive and Shared Drive for files within your Workspace. As an Admin, you can then take owner ship of files in user My Drives and delete those files if necessary. GAT+ is quite extensive. I use GAT+'s reporting capabilities for instance to see what files are being shared between students that do not have a teacher shared as well. It allows me to detect chat documents quite easily.
Or I can search for contacts in users' personal contacts that had something changed, or groups they've created and modify them as needed, without having to go to each user individually.
In your case, you would be able to search for the document names take ownership, remove other permissions in one go (in which case they'd be moved to your My Drive) and either delete them or store them for further investigation.