Hi @Anonymous ,
Nice progess and good work :-). Also good screenshot to explain your logic.
Did you added the column called 'Last Checked By Flow' (DateTime) in your library?
If not then go to SharePoint library. Click the settings > library settings
OR Click the add new column from your SharePoint library
Create column : Last Checked By Flow : Type: Date and Time
See below SharePoint library columns.

Once you created the column then open your flow 'Get File properties'
You should be able to retrieve the column 'Last Checked By Flow'.
Please see below.

Hope it make sense. Any help let me know.
Thanks